"the two protuberances which form the rump in men and animals," c. 1300, probably from Old English buttuc "end, short piece of land," from Proto-Germanic *butaz, from PIE root *bhau- "to strike," thus related to butt (n.1).
"posterior, buttocks, rump," from mid-15c. in cookery, in reference to animal parts, probably from or related to butt (n.1) "thick end," or short for buttock. In modern use chiefly of humans, probably an independent derivation, attested by c. 1860 in U.S. slang.
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Definitions of buttock from WordNet
either of the two large fleshy masses of muscular tissue that form the human rump;