Cossack

1. 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 neighbouring countries, that eventually settled in parts of the Polish-Lithuanian Commonwealth and the Russian tsarist Empire and constituted a military caste, particularly in areas now comprising southern Russia and Ukraine.
2. A member of a military unit (typically cavalry, originally recruited exclusively from the above).
3. A mercenary, a violent thug: a regular or irregular soldier, police officer or security guard employed to persecute or oppress disfavoured groups, such as Jews or (striking) workers.

Plural Cossacks