Description of a booty that "pops" out from the waist, like the top of a muffin.
"Damn, I'd love to fall asleep on Molly Jane's muffin ass."
A big, fluffy ass. Also, a person who has a big, fluffy ass.
"My wife wants it--a picture of me naked, 'cause she says I have a wonderful muffin-ass." - Sol Rosenberg