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1.
A woman with a very flat chest, such that it resembles her back. Only the nipples give it away.
The girl at the party was pretty, pity about her two backs.
by DesDDD September 24, 2007
 
2.
A "twoback" is a woman with no visibly protruding breasts. See also: "flat as a board."
Lucy the twoback rarely goes on second dates.

"Is that a stop sign or a twoback?"
by snowshoesny September 26, 2009