In prison, the act of concealing objects in your anus or in the crack of your ass.
-Yo, Bill. How did you get those cigarettes in here, man?-
-I keister stashed 'em, dude.-
A popular hiding place for contraband such as drugs, especially in prison. The anus. Usually the contraband is wrapped in a condom.
Johnny kept is smack in a keister stash.