Green’s Dictionary of Slang

keister v.

also keester
[keister n. (5)]
(US, esp. prison)

1. [1930s+] (also ass-keister) to hide something in the rectum; also as vtr., to hide something in someone else's anus.

2. [1930s+] to sodomize.

3. [1960s+] to betray, to harm [fig. use of sense 2].