[cos.sack] n, often cap [Pol & Ukrainian kozak, of Turkic origin; akin to Volga Tatar kazak free person] (1589): a member of a group of frontiersmen of southern Russia organized as cavalry in the czarist army
COSSACK
Meaning of COSSACK in English
Merriam-Webster English vocab. Английский словарь Merriam Webster. 2012