vestigial buttsack

When a person is born with the parts to form a butt (butt cheeks) but no flesh ever properly forms in it, so that the prson doesn't have the natural roundness of a butt and are therefore forced to sit on the extension of their thighs for lack of a natural butt. Or in simpler terms- lack of butt meat.

Hold on, is that Crystals's thighs or just her vestigial buttsack that's creeping from the top of her jeans when she bends over?