The rear view of the vagina that is made available as a result of a skirt being too short.
John's new girl was such a skank that her back vag was showing at the restaurant.

Also: Dude, check out the back vag at eight o'clock.
by lisanik October 27, 2008
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When a persons ass crack is showing above the waist of their pants creating a vagina like crack on their backside.
Mike "Dude, that girl in front of us in economics totally had a back-vag going on."
Al "I know man. It was so hot."
Mike "Dude! She was 300 pounds at least!"
Al "I know."
by Adam Woodard February 20, 2007
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