We don't have synonyms for kosak yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are kodak, kojak and kozak.
(n) :
Obsolete form of Cossack. [A member or descendant of an originally (semi-)nomadic population of Eastern Europe and the adjacent parts of Asia, formed in part of runaways from neighbouring countries, that eventually settled in parts of the Polish-Lithuanian Commonwealth and the Russian tsarist Empire and constituted a military caste, particularly in areas now comprising southern Russia and Ukraine.]
(n) :
Obsolete form of Cossack. [A member or descendant of an originally (semi-)nomadic population of Eastern Europe and the adjacent parts of Asia, formed in part of runaways from neighbouring countries, that eventually settled in parts of the Polish-Lithuanian Commonwealth and the Russian tsarist Empire and constituted a military caste, particularly in areas now comprising southern Russia and Ukraine.]