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 in a negative connotation.
In 2268, Chekov called Korax a Cossack after the Klingon claimed that Earthers reminded him of Regulan blood worms. (TOS: "The Trouble with Tribbles")
Later, Chekov also referred to the Providers of Triskelion as Cossacks. (TOS: "The Gamesters of Triskelion")
Additionally, Chekov 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")