(esp. in czarist Russia) a person belonging to any of certain groups of Slavs living chiefly in the southern part of Russia in Europe and forming an elite corps of horsemen.
Origin: 1590–1600; < Polish kozak or Ukrainian kozák, ult. < a Turkic word taken to mean “adventurer, freebooter,” adopted as an ethnic name by Turkic tribal groups of the Eurasian steppes