Cossacks - military estate in Russia and Ukraine in the XVI - early XX century. at. The Cossack (Cossacks, formerly Cossacks derives from the Turkic language and means free man, is akin to Kazakhs) - the South Russian and Ukrainian subethnoses that appeared in the 15th century on the Lithuanian-Crimean border (passing along the Dnieper River).By the 18th century, Cossack hosts in the Russian Empire occupied effective buffer zones on its borders.