A Cossack was a member of communities of horsemen who lived on the open grasslands of what is now southern Russia and Ukraine, from about the 1400s onwards. They were not a country and not exactly one people — they were free groups, many of them people who had run away from serfdom to live on the frontier, and they governed themselves. Later the Russian tsars used them as soldiers, and that is where their famous picture comes from: superb riders, tall fur hats, and the dance where a man squats low, folds his arms, and kicks his legs out in front. In English, 'Cossack' is almost always historical — you meet it in novels, films, history classes, and in the names of things borrowed from that look ('Cossack hat', 'Cossack dance'). It is capitalised, because it names a people. Be careful with the feeling attached to it: in Russian and Ukrainian history the Cossacks are heroes of freedom to some and, because tsarist Cossack troops were used to break up crowds and attack Jewish communities, a frightening memory to others. English speakers today mostly use the word for the image — horses, dancing, the wide grassland — without knowing either half of that story.