What's the difference between i.e. and e.g.?
"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).
[fr British dialect kist or German Kiste, ''chest, box,'' transferred to the buttocks perhaps by the pickpocket sense or by the notion that something may be concealed in the rectum]