keister (n.) Look up keister at Dictionary.com
"buttocks," 1931, perhaps transferred from underworld meaning "safe, strongbox" (1914), earlier "a burglar's toolkit that can be locked" (1881); probably from British dialect kist (northern form of chest) or its German cognate Kiste "chest, box." The connection may be via pickpocket slang sense of "rear trouser pocket" (1930s).