The act of a womans or mans underwear or bathingsuit getting swallowed by their buttox. In worse situations it may be referred to as a starving bum. Usually this happens for larger ladies that wear too small of bathinsuits because they think its attractive to look at cottage cheese.
<duDe> OMG@! look at the hungry bum
<man> yea dude, her fucking ass swallowed her whole bathingsuit
When a person's pants/jeans/shorts are riding up the butt in an unattractive fashion requiring a "manual release" of sorts - usually happens to overweight women (but not exclusively). It looks like the bum is literally eating the pants.
Jason: "Check out the hungry bum on the chick in white pants."