Zafu Via huggermugger These delightful Japanese meditation pillows are great from when you want to rest your keister.
Now, Tuttle, set down that there keister of yours—that gripsack—and listen.
"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]