A hot girl's vagina.
Very selective on who's dick is permitted through the membrane (her pussy lips.)
Will often deny nervous, non-jock types. The membrane judges largely on physical appearance, not on the pretense of a good personality or wit.
Guy 1- So did you sleep with Elena in the end?
Guy 2- No, she had a selectively permeable membrane.
Guy 1- ... So she turned you down?
Guy 2- Yeah.
Guy 1- Well just say that then!
Guy 2- I needed to make an example for a definition for urban dictionary, okay?!
Guy 1- Oh, right... Wait a minute, am I on TV?!
A slang word combining the words ace and prime, two words used for describing something that is awesome. As such, primeace is (at least) twice as awesome.
Oh yeah, and Chuck Norris uses it.
The lower back of the lady over there is pretty primeace.
Barney Stinson is primeace.
Cheezburgers are really primeace!