Cossack

noun4

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. •
   (disapproving) 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.

   • 1865 The Coal and Iron Police are authorized by the Pennsylvania General Assembly to maintain order in the coal fields. The miners referred to these private policemen as "Cossacks" and "Yellow Dogs." — 1865
4. •
   (no longer used) A Ukrainian.