Cossack was a term used in the Russian language to romantically describe the frontiersmen of southern Russia. The term was often used by Pavel Chekov.
One way Chekov used the term was to describe the Klingons in a negative connotation. He once referred to Korax as a Cossack after Korax noted that Earthers reminded him of Regulan blood worms. (TOS: "The Trouble with Tribbles")
Chekov also referred to the Providers of Triskelion as Cossacks. (TOS: "The Gamesters of Triskelion")
Additionally, 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")
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 imaginary brother Piotr Chekov. (TOS: "Day of the Dove")