Green’s Dictionary of Slang

keister v.

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

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

[US]Bentley & Corbett Prison Sl. 76: Keyster Literally, keyster refers to a person’s buttocks. […] To smuggle drugs in one’s anus would be to keyster them.
[US]K. Scott Monster (1994) 324: I keistered it [i.e. a knife] and brought it from L.A. Country jail.
[US]J. Lerner You Got Nothing Coming 19: There will be a cavity check [...] for the benefit of any ignorant motherfucker that thinks he can ass-keister a hypo or crack pipe or even a naked photo of his sister Kate doing the shimmy.
[US]Mad mag. July 41: What about the meds? Safely keistered, my friend.
[US]C. Goffard Snitch Jacket 100: ‘Happy Easter the nail and plank’ meant keister the shank.
[US]J. Stahl Pain Killers 166: See that [...] she keistered him! Everybody think’s he’s a bitch, meanwhile the Flip probably made a grand.
[US]J. Stahl Happy Mutant Baby Pills 16: Because of my own decade and a half imbibing kiestered Mexican tar, I got some kind of heinous [...] parastite .

2. to sodomize.

[US]J. Ellroy ‘Jungletown Jihad’ in Destination: Morgue! (2004) 352: He keesters cats while his comrades catch it on camera.

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

[US](con. 1964–8) J. Ellroy Cold Six Thousand 413: So he celebrates en route to Leavenworth. We keester Howard Hughes, and jimmy packs his pj’s for the pen.