Buttocks
来自Big Physics
Middle English, apparently from butt3 (although the earliest evidence is from a later date) + -ock.
etymonline
buttocks (n.)
"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).
