keister (n.)

"buttocks," 1931, perhaps transferred from the same word in an underworld meaning "safe, strongbox" (1914), earlier "a burglar's toolkit that can be locked" (1881); probably from British dialect kist (northern form of chest (n.)) or its German cognate Kiste "chest, box." The connection of the word to the body part might be via the pickpocket slang sense of "rear trouser pocket" (1930s).