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1.
a verb for storing contraband in your rectum, common in prison
“I don’t understand how there’s so much drugs in the prisons. How do they get it all in?” Gemma asked.
“Mostly keystering.”
“What’s that?”
“It’s packed in balloons or condoms. The vistors insert them into their bodies, take them out during the visits and the prisoners insert them into their bodies at visitation. Women visitors have two places they can insert them, and men one. A prisoner who can store a lot inside himself is called a mule. He’ll get paid by the gangs to receive drugs through visitation. Sometimes the packages burst and the mule dies of an overdose or is hospitalised.”
-from Jonsjailjournal
by Shaun Attwood May 14, 2008
 
2.
A person that holds the key(s). Whether it be access to something to place or essential information.
1. Ashley has the key to my heart. She's my keyster.
2. Hey Jim, those emails you forwarded about my soon to ex girlfriend really did come in handy. Dude, you're a keyster for sure.
3. DJ Blow's a keyster. He always get us into the hottest parties.
by Optimus Primer April 12, 2010
 
3.
To visually undress and 'fuck' somebody; to check someone's 'assets' out intensely, most commonly as they pass
As the milf walked by the two teenage males, they thoroughly keystered her large ass and double-D's.
by Violet! September 05, 2008