when one's bra is too small, and one or both boobs are poking over the top of the cup. there are varying degrees of overboob; and when a severe case affects both boobs, it is called quadriboob.
I haven't worn this bra in a while, I must've outgrown it... I've had overboob all day long and no one told me!
Much like the "sideboob" the "Overboob" is the part of a woman's breast that is exposed or casually visible by others.
I think she needs a bigger bra. Look at all that "overboob" hangin' out.