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!
This shirt has shrunk so much in the wash I'm practically showing off my nethage.