When a womans butt is saggy, flat and generally offensive. Such butts are neither round and supple nor has any hope of such glory. Dumpbutts look just as they sound. The cheeks often have a square droop to them as though they had a load of poop in their pants but just kept on going without changing thier pants. Frankly it looks like their butt is a diaper of flesh and cottage cheeze. When a dumpbutt is on the go it looks like their cheeks are flapping because of flatulance.
"My lord, is that a dead animal in her pants or does she have the worst dumpbutt that I have ever seen?!"
"Everyone run, some girl on the the dance floor just broke out the booty dancing and her dumpbutt is pwning everyone wild."
Hey, You! Yeah, the only girl in the room with a Dumpbutt! Yeah you know who you are!