We do not currently know of any antonyms for Cossack.
The noun Cossack is defined as:
A member or descendant of an originally (semi-)nomadic population of Eastern Europe and the adjacent parts of Asia, formed in part of runaways from the neighbouring countries, that eventually settled in parts of the Polish-Lithuanian Commonwealth and the Russian tsarist Empire (where they constituted a legendary military caste), particularly in areas now comprising southern Russia and Ukraine.
