i) The visible portion of a woman's vulva,
from behind, when she bends over.
ii) The amount the vulva protrudes (sticks out) downwards when a woman bends over.
iii) Cameltoe, as seen from behind, when a woman bends over.
Women who have a plump vulva with fleshy labia have a lot of underhang, while women with a flat vulva and lean labia have very little underhang.
All the above definitions apply to both clothed or nude women.