Goombutt
A butt with a shape similar to that of a Goomba, the brown, mushroom-like enemy from the Super Mario video game franchise. Like a trapezoid with rounded corners, the butt is wider at the bottom than the top and horizontally flat across the lower edge. In form fitting pants, rather than looking like a pair of melons, the goombutt looks more like an underfilled water balloon sitting on a table.
I thought she looked good, but then she turned around and saw that she's got a goombutt. That chick needs to get her ass to the gym.