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.
A shirt that looks like it was made for a child or infant that is sometimes worn by teens and adults. Should never be worn by an obese female or any male. May be worn by sexy females, especially to show off their neathage / under boob.
Dude 1: Did you see that chick wearing that baby GAP shirt before?
Dude 2: Yeah that was sexy as hell!
Dude 1: Yeah it accentuated her neathage so well!!