Of women, when one's (bra) cup runneth over slightly, marring slightly the smooth convex curve of bra and boob-top. A tiny amount of overboobage is intriguing, but excessive overboobage leads to the much-feared four-boob effect. Overboobage is most prevalent in push-up bras, but can be found in any over-tight bra that is not full-coverage.
Bring me another bra. This one's cups are too small, and I'm getting severe overboobage.