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!
The cleavage seen when a women is wearing a very high cut top. Basically, the tops of the breasts will be covered but from about an inch below the nipples and down will be bare. The word is a portmanteau of "cleavage" and "nether" (meaning under, or below).
Did you see the nethage on that girl back there? She was practically topless!