The fat bulge resulting from a person (usually women) who wears pants that are too tight along with a halter/tube top.
That fat bulge looks somewhat like the top of a muffin, hence the name.
"I guess that fat chick used the money on an extra Big Mac instead of bigger pants and a whole shirt."
Respectable woman: "What the hell is that fat bitch thinking going out in public like that?!"
I would use an example with "muffin-top" in it, but what the hell type of urban slang is muffin-top? Corporate America is not urban