Definition from Wiktionary, the free dictionary
underboob (plural underboobs)
- (colloquial) The lower part of a woman's breast, especially when uncovered or revealed by clothing.
2009, Zack Parsons, Your Next-door Neighbor is a Dragon, page 69:
- He beat me almost every time with the girl with the visible underboobs, although I got him once or twice with the guy wielding a sword bigger than his leg.
the lower part of a woman's breast