1 definition by T-TIMMEH!

Top Definition
The involuntary reflex of clenching your butt cheeks together whenever you hear a noise behind you.

Done in prison to mitigate the possibility of anal rape.
1) Wow! Amanda's ass clenched up big time just now!
2) Yeah, she's got total prison reflex.
by T-TIMMEH! December 22, 2010

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