The underside of a woman's brests when she wears a shirt cut up to the bottom of them.
Wow! If that girl wore her shirt and higher, her neathage would be showing her nipples
Whereas, cleavage implies the top view of a woman's breast, "neathage" implies the bottom view. This is usually accomplished by a female wearing a small shirt, or lifting up her shirt so that not only her abdominal is exposed, but also the bottom of her breasts. Neathage is also possible when a girl has a hand bra, covering the nipples but exposing her cleavage and neathage.
See also: under boob
Dude 1: Yo did you see that chick?
Dude 2: You mean the hot one wearing a baby GAP shirt?
Dude 1: Yeah man you could see her neathage!
Dude 2: Hell yeah dude!