Underboob is a view of the bottom of her tits caused by the short design of her top by intension. The underboob might only show you a very small region of the very bottom of her tits or might go up almost under her nipples. Nipples are not part of an underboob. Underboobs are a kind of upside-down cleavage.
Important: An underboob is intended by her and requires dedicated underboob-friendly clothing. If the view to the bottom of her boobs is caused by the a malfunction of her clothing, it is not an underboob but an upblouse!
Function of an underboob: Wearing underboob-friendly clothes allows women to display part of their tits in public while fully dressed. It also allows women to blame a tits oops for any photos taken of their underboobs, while in fact this was exactly what they wanted. If a woman showing her underboobs bents over, the audience will be able to see her full tits hanging down.
Underboob voyeured on a beach