Flat-chested, adj (flåt chês-tæd) The condition in which the breasts of an adult human female are small to nonexistent. Flat-chested women are eccentric child-like beauties. While they lack the visibility of breasts, they often look younger than they truly are, and are easier to hit on from their low self-esteem. Flat-chestedness is medically defined as any cup size "A" or lower. med A condition which, when it afflicts a female, is strangely pleasant to experience fucking as a male. And can they help it? NO. I myself am fine with that, I love the flat tits.