One with which whom's ass has become large. Not by means of fitness but by means of laziness and unhealthy eating.
The term originated from disputes about female's butts and whether they were nice or not. just because it was large doesn't make it nice because some girls get large butts from sitting on the couch and stuffing their face with "Cheetos."
That girl doesn't have a firm ass, she has a Cheeto Butt.