A mouthbreathing illiterate moron's mishearing and subsequent misspelling or mispronunciation of the word "comeuppance." The combination of "cum" and "muffins" adds hilarity to the usage of the term, as the word is typically placed in the syntax as an object to be received, as a result of karma, by a person perceived to have committed a wrong.
A:hey whatcha eating
B: dude im working on my third big mac
A:what of the week?
B: no of the day!
A:wow what a chubbamuffin
or
A:i dont feel so good man
B:why? you ok man
A: i dont know man i was feeling like a chubbamuffin and a gallon of ice cream.