Where a vagina's lips shut so rapidly that they make a clapping noise. Suggestive that they are big flappy flaps and that a large penis has been withdrawn at some huge speed.

A kind of vaginal sonic boom. You need damn big flaps to make a full fanny clap.
<CLAP!>
Dude: "What da fuck was that, woman!?"
Chick: "Damn muthafucka! You took you damn dick out too fast for me, that was my fanny clap Nigga!"
by Wizards Sleeve January 28, 2007
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