Tank Ass

Generally, a wide, thick butt that doesn't have reasonable curvature (A.K.A. bubble butt) or shapely definition.

A large butt that lacks roundness (may be flat in places), although it may have depth, and a shelf.

My girl gained a bit of weight after the baby, and it all went to her hips. Her booty used to be round, but now it's more of a tank ass now.