Definition of kosak:
(n) :
A surname.
(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.]