Word Origin & History
keister
"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 (c.1300, northern form of chest, from O.N. kista) or its Ger. cognate Kiste "chest, box." The connection may be via pickpocket slang sense of "rear trouser pocket" (1930s).
[kee-ster] 
stər