Hungry bum, is a state that your bum goes into. Essentially, it is when one's ass hole tries to swallow that person's undergarments. This state can aslo happen (in some cases) where there is execcive bum hair. The ass hole attemps to devour the hair.
<dude> That milf over there has got a hungry bum eating her whole fuckin bathing suit!!!
<man> yea dude, her fucking ass swallowed her whole bathingsuit
Sam: "Yikes, that's just wrong."