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.
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.
Craig: That my friend is some grade A UnderBum.
by Hector the molester July 28, 2010
almost like a double chin
"does my underbum look big in this?"
"does this skirt emphasize my underbum?
"underbum is way rank"
"does my underbum look big in this?"
"does this skirt emphasize my underbum?
"underbum is way rank"
by Chickenpastababy October 13, 2011