keister
From Longman Dictionary of Contemporary Englishkeisterkeis‧ter /ˈkaɪstə $ ˈkiːstər/ noun [countable] HBH American English spoken your buttocks (=the part of your body that you sit on)Examples from the Corpuskeister• Barry fell off his chair and landed on his keister.Origin keister (1800-1900) Probably from German kiste “box”