Noun
(colloquial) The roll of flesh that bulges over the top of excessively tight trousers.
Used other than figuratively or idiomatically: see muffin, top; the top of a muffin, sometimes removed to be eaten separately.
Source: en.wiktionary.orgRed flag of the eating disorder: the muffin. Keep your eye on the ladies with the muffins... and sometimes I'll just eat the muffin top. Janeane Garofalo
My saddlebags are why Spanx exist! Now that I have a baby I also have a muffin top. Sara Blakely