Cossack is a term used in the Russian language to romantically describe the frontiersmen of southern Russia. The term was often used by Pavel Chekov.
Chekov once referred to American frontiersmen of the 19th century as "western Cossacks" who had little more than poisonous snakes and cactus plants. (TOS: "Spectre of the Gun")
He also used it in a negative connotation to describe the Klingons. He once referred to Korax as a Cossack after Korax noted that Earthers reminded him of Regulan blood worms. (TOS: "The Trouble with Tribbles")
He also referred to the Providers of Triskelion as Cossacks. (TOS: "The Gamesters of Triskelion")
While under the influence of the Beta XII-A entity, Chekov referred to the entire Klingon race as Cossacks as well as "filthy Klingon murderers" for killing his fictional brother Piotr Chekov. (TOS: "Day of the Dove")