This is not perhaps what Putin meant with it, but why he used the word at all. Hope it is still on-topic for the question then. There are two reasons for the word "genocide" perhaps:
- This statement about genocide may bear some truth, not in the sense that it is happening, but in the sense that it could happen since the Ukrainians could take revenge on the forced Russian mass immigration into the area in earlier times so that the Russians in the Ukraine (also civilians) are really in danger now. If you cannot fight Russia on the military level, there could arise a civil revenge, This of course for a reason. There was a kind of genocide that the Soviet-Russians committed on Ukrainian cossacks during Holodomor which then thinned out the lands so much that Russians could settle. And even before Holodomor, in 1920, those Ukrainian cossacks who had fought against the Bolsheviks and who had fled to the Russian cossacks in the east had to flee from the Bolshevik invadors. And in even earlier times, there were also few cases when Ukrainian cossacks had fought against the Russians (mostly, they were loyal!) and had to flee from their land afterwards (without the Cossacks, Russia would not be as big as it is now anyway, they had a big share). Putting all of this together, the events now seem like a Stalinist long-term plan that thrives to be fulfilled now.
- Saying such a thing is then also a rhetorical trick to put in a very high claim, turning around the (hi-)story by 180 degrees (since it was Soviet Russia that enforced Holodomor on the Ukrainian Cossacks), so that even after a discussion you still do not reach the original direction any more. It is like when the buyer bids a very cheap price while the seller demands the right and fair price at first, and they might meet in the middle, they will perhaps not end up at the higher entry price anymore. Same with lawsuits asking for many millions to get at least a share of it.
And if there is such violence, is there also genocidal activity against ethnic Ukrainians by ethnic Russians?
At least there have been such threats in the historical past, which makes it politically relevant for now as well, see the following.
History and end
I took this from another question that was closed, Can the aggressions of Russia against Ukrainian territory be understood in a politico-historical light?, therefore, it does not fully answer the question here in each sentence. It just gives more grip to the points above.
From searching for a map of "Moscow Russian expansion", you find out quickly that it all started from tiny Moscow area, taken from here but you will find many more of course.
Here is a longer read-through of the cossaks wiki, TL/DR. Read just the wrap up at the end.
The cossacks were a name for a kind of border police of the Slavic Russian-Ukrainian area. They were Ukrainians or Russians, but those cossacks that are relevant for the Ukraine nowadays are of course the Ukrainian cossacks.
- Cossacks may play a key role in marking Russia as the main aggressor with too expansionist aims and Crimea only as a satellite area since the cossacks that are relevant lived in rural Ukraine and eastwards around the Ukraine (Don and Kuban) where nowadays are Russians.
Map of the "Wild fields" / Wilderness which was colonized by cossacks in the 15th century. Map is linked on the main wiki page about cossacks.
Wild Fields / wilderness area (nowadays eastern Ukraine):
According to Ukrainian historian Vitaliy Shcherbak the term [Wildfields] appeared sometime in the 15th century for territory between the Dniester and mid-Volga when colonization of the region by Zaporozhian Cossacks started.2 Shcherbak notes that the term's contemporaries, such as Michalo Lituanus,3 Blaise de Vigenère, and Józef Wereszczyński [pl; ru],5 wrote about the great natural riches of the steppes and the Dnieper basin.2
For centuries, the region was only sparsely populated by various nomadic groups such as Scythians, Alans, Huns, Bulgars, Pechenegs, Kipchaks, Turco-Mongols, Tatars and Nogais.
Read the full text about Cossacks, also take a short glimpse at a famous Russian cossack Yemelyan Pugachev to see that not only Ukrainian cossacks play a role here. Russian Cossacks would normally have to have had their own homeland in the east of Ukraine as they had it from 1918 to 1920, but Bolshevik Russia took it back and took their independence.
By early 1920, the Bolsheviks had overrun most of the Kuban region and the soldiers of the Volunteer Army [the Volunteer Army that was fighting against the Bolsheviks], along with their families had fled to Crimea.
From Kuban cossacks
where you also find the map:
And those Ukrainian cossacks on the Ukraine side who lived in the rural areas and who were not loyal to Russia had to flee to the Russian cossacks or starved or froze to death during Holodomor later, so that Russians could settle the area.
Among others, from the cossacks wiki:
The Zaporizhian Cossacks became particularly strong in the first quarter of the 17th century under the leadership of hetman Petro Konashevych-Sahaidachny, who launched successful campaigns against the Tatars and Turks. Tsar Boris Godunov had incurred the hatred of Ukrainian Cossacks by ordering the Don Cossacks to drive away from the Don all the Ukrainian Cossacks fleeing the failed uprisings of the 1590s. This contributed to the Ukrainian Cossacks' willingness to fight against him. In 1604, 2000 Zaporizhian Cossacks fought on the side of the Polish-Lithuanian Commonwealth, and their proposal for the Tsar (Dmitri I), against the Muscovite army. By September 1604, Dmitri I had gathered a force of 2500 men, of whom 1400 were Cossacks. Two thirds of these "cossacks", however, were in fact Ukrainian civilians, only 500 being professional Ukrainian Cossacks. On July 4, 1610, 4000 Ukrainian Cossacks fought in the Battle of Klushino, on the side of the Polish-Lithuanian Commonwealth. They helped to defeat a combined Muscovite-Swedish army and facilitate the occupation of Moscow from 1610 to 1611, riding into Moscow with Stanisław Żółkiewski. --> This shows that Ukraine and cossacks are strongly linked, that theses cossacks also fought the Tatars and Turks, and that they tried to get their own independent homeland.
[Black Sea, Azov and Danubian Sich Cossacks:] But the majority of Zaporizhian Cossacks, particularly the
Ukrainian-speaking Eastern Orthodox, remained loyal to Russia despite
Sich destruction. This group became known as the Black Sea Cossacks.
Both Azov and Black Sea Cossacks were resettled to colonize the Kuban
steppe, a crucial foothold for Russian expansion in the Caucasus. --> Cossacks helped Russia in expanding into the Caucasus
[-- Russian Cossacks: ] Cossacks served as border guards and protectors of towns, forts, settlements, and trading posts. They performed policing functions on the frontiers, and also came to represent an integral part of the Russian army. In the 16th century, to protect the borderland area from Tatar invasions, Cossacks carried out sentry and patrol duties, guarding against Crimean Tatars and nomads of the Nogai Horde in the steppe region.
The most popular weapons of the Cossack cavalrymen were the sabre, or shashka, and the long spear.
From the 16th to 19th centuries, Russian Cossacks played a key role in the expansion of the Russian Empire into Siberia (particularly by Yermak Timofeyevich), the Caucasus, and Central Asia. Cossacks also served as guides to most Russian expeditions of civil and military geographers and surveyors, traders, and explorers. In 1648, the Russian Cossack Semyon Dezhnyov discovered a passage between North America and Asia. Cossack units played a role in many wars in the 17th, 18th, and 19th centuries, including the Russo-Turkish Wars, the Russo-Persian Wars, and the annexation of Central Asia.
[-- Don Cossacks, east of Ukrainian border:] The gene pool comprises mainly the East Slavic component, with a significant Ukrainian contribution. There is no influence of the peoples of the Caucasus; and the steppe populations, represented by the Nogais, have only limited impact. --> Ukraine and cossacks are strongly linked
[-- Kuban Cossacks, southeast of Ukrainian border, directly touching Crimea:] are Cossacks who live in the Kuban region of Russia. Although many Cossack groups came to inhabit the Western North Caucasus, most of the Kuban Cossacks are descendants of the Black Sea Cossack Host (originally the Zaporozhian Cossacks), and the Caucasus Line Cossack Host.
A distinguishing feature is the Chupryna or Oseledets hairstyle, a roach haircut popular among some Kubanians. This tradition traces back to the Zaporizhian Sich. --> one could guess that Crimea was rather Cossackian than Russian or Ukrainian?
In late 1943, the Reich Ministry for the Occupied Eastern Territories and Wehrmacht headquarters issued a joint proclamation promising the Cossacks independence once their homelands were “liberated” from the Red Army. --> there is a homeland that was ruled by expansionist Russia
[Cossacks in the Soviet Union:] The Bolsheviks embarked on a genocidal policy of “de-Cossackization”, intended to end the Cossack threat to the Soviet regime. This was pursued through resettlement, widespread executions of Cossack veterans from the White armies, and favoring the outlanders within the Cossack hosts. Ultimately, the de-Cossackization campaign led to a renewed rebellion among Cossacks in Soviet-occupied districts, and produced a new round of setbacks for the Red Army in 1919.
In 1932–1933, another famine, known as the Holodomor, devastated
Ukraine and some parts of South Russia, causing a population decline
of about 20–30%. While urban areas were less affected, the decline was
even higher in the rural areas, populated largely by ethnic Cossacks.
Robert Conquest estimates the number of famine-related deaths in the Northern Caucasus at about one million. Government officials expropriated grain and other produce from rural Cossack families, leaving them to starve and die. Many families were forced from their homes in the severe winter and froze to death. --> Many Cossacks lived eastwards from eastern Ukraine
Cossack wrap up:
The cossack history shows how expansionist Russia used the cossack's help for its expansion only to deport, suppress and diminish them in Soviet time (“de-Cossackization”, Holodomor). Those cossacks who had fought for their own homeland which the Germans had promised were also deported to die. Since Russia is a deeply expansionist state from the start and since Cossacks were the rural population of Ukrainia way before the Russians came, it is perhaps not a question of Crimea being historically Russian, but about whether this border region and eastern Ukraine would not rather be land of the cossacks, which is likely in the Ukrainian area where now Russians live. Anything else accepts the cruelties of the Stalinist Regime.
Crimea / Eastern Ukraine wrap up
Having Crimea still as Ukrainian would not bring back those Cossacks, though, when mainly Russians live there nowadays. It is politically not realistic to resettle Russians of Eastern Ukraine and Crimea elsewhere nowadays unless Ukrainians start a civil war against them (which then would likely become war against Russia). Weighing everything up, it comes all down to whether the expansionist Russia with the Stalinist repressions are bad enough to be guilty up to nowadays or not, and this question does not need to be answered. The question is already politically decided by the mere political and military power of Russia. Seems like only the NATO is more powerful which then makes it clear why Ukraine tries to become its member.
Full history of Crimea in keywords
See History of Crimea for details, list of cultures:
Per Haak et al. (2015), the Yamnaya contribution in the modern populations of Eastern Europe ranges from 46.8% among Russians to 42.8% in Ukrainians..
Then Tauri and Scythians, Greeks, "Roman Empire" (Crimea was invaded or occupied successively by the Goths (AD 250), the Huns (376), the Bulgars (4th–8th century), the Khazars (8th century). Crimean Gothic ... spoken by the Crimean Goths in some isolated locations in Crimea until the late 18th century), Rus' and Byzantium, Mongol invasion, Ottomans / Tatars, Cossacks, Ottomans / Tatars, Russians / Greeks / Tatars, Stalin repressions against Tatars + Greeks, Kuban cossacks, Soviet Russian settlement.
The statement on genocide may aim at blurring the genocide of Soviet-Russia on the Ukrainian cossacks during Holodomor (and other suppressions during earlier cases when those Ukrainian cossacks that were not loyal to Russia had to flee). Talking about "genocide" makes the highest possible claim in favor of the Russians, as history was the other way round: Ukrainian cossacks were those that suffered. Talking about risks of "genocide" is still not just a saying out of thin air since with this historically grown background, it may well happen that civilians of both parties are really in danger, and the ethnic Russians much more than the ethnic Ukrainians, in a hidden civil war against the civilians which would not be astonishing. Perhaps, the only way to reach security against such far-right threats against civilians is to fully separate Ukrainians from Russians - something that seems to and needs to happen the more both parties get alienated from each other.