A person, male or female, who is an expert in matters pertaining to the genital region of the human body, particularly regarding sexual concerns. Can be used in a derogatory fashion or to compliment sexual prowess.
From Pavement's "Here":

"Let's spend our last

Quarter stance randomly

Go down to the outlet once again

Painted portrait of minions and slaves

Crotch mavens and one night plays

Are they the only ones who laugh "
by summerbabe33 October 30, 2011
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