Lighter, Historical Dicitionary of American Slang:
keister n. [< G Kiste 'box, case, chest' also (slang) 'rump'] Also kiester, keaster, keester, keyster.
2.a the posteriors; BUTT, 1.a.; (also) the anus or recturm. -- also use in fig. senses. Also keisters.
1931 AS VI (Aug.) 439 [ref. to ca. 1925]: Keister, n. A satchel; also what one sits on.
1938 "Justinian" Amer. Sex. 27: Keyster, n. Posterior; buttocks. Becoming a euphemism, 1925 --
1983 ABS World News (ABC-TV) (Jan. 10):A newspaper quoted .. President Reagan as saying, "I've had it up to my keister with these [news] leaks."
The Cassell Dictionary of Slang:
keister/keaster/keister/keyster/kiester n. (US)
1. [late 19C+] a suit case, a satchel, a handbag, a saleman's sample case
2. [late 19C+] a burglar's bag of safe- or house-breaking tools
3. [1910s-1950s] (Und.) a safe, a strongbox
4. [1930s+] the anus, the buttocks
5. [1930s-1960s] sexual intercurse with a woman
6. [1930s-1960s] (esp. prison) anal copulation; thus keister bandit, a homosexual
7. [1940s-1960s] a prison
8. [1960s+] one's self (cf. ARSE n.1) [Ger. kiste, a box, a case + Ger. sl. the rump]