Cossaks were feared Russian military elite, but cossaks is now a negative term used to describe Russians or Eastern Europeans. The hate might come from the fact that they fought with the Nazi's during WWII and before then generally hated Jews.
"He tried to posion me the dirty Cossak!" - Lenny from RocknRolla
"My Grammy never gave gifts, she was too busy being raped by cossacks" - Woody Allen in Annie Hall