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1. When a female wears really short shorts and reveals the underside of her ass to passers by.

2. God's gift to ass men.

3. Also known as ass cleavage.
Adam: holy shit that chicks ass is hanging out! I think my dick just moved.

Craig: That my friend is some grade A UnderBum.
by Hector the molester July 28, 2010
14 3
When a female has a second bum
almost like a double chin

"does my underbum look big in this?"

"does this skirt emphasize my underbum?

"underbum is way rank"
by Chickenpastababy October 13, 2011
13 5