buttock

Từ điển Oxford Advanced Learner 8th

but·tock [buttock buttocks] BrE [ˈbʌtək] NAmE [ˈbʌtək] noun usually plural
either of the two round soft parts at the top of a person's legs
•The beating had left red weals on his buttocks (= on his bottom).

Word Origin:
Old English buttuc, probably from the base of ↑butt ‘thicker end of something’ + -ock.