1580s (earlier buffel, 1510s, from Middle French), from Portuguese bufalo "water buffalo," from Medieval Latin bufalus, variant of Latin bubalus "wild ox," from Greek boubalos "buffalo," originally the name of a kind of African antelope, later used of a type of domesticated ox in southern Asia and the Mediterranean lands, a word of uncertain origin. It appears to contain bous "ox, cow" (from PIE root *gwou- "ox, bull, cow"), but this is perhaps a folk-etymology association.
Wrongly applied since 1630s to the American bison. The other Germanic words (Dutch buffel, German Büffel, Danish böffel, etc.) are from French; from Medieval Latin come Russian buivolu, Polish bujwoł, Bulgarian bivol, etc. Buffalo gnat is recorded from 1822. Buffalo chip "dung of the American bison," used for fuel on the U.S. plains, is from 1840.