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.
Colloquial term for someone who has done something bad in the past and now the bad karma is catching up with them and punishing them. The punishment one recieves when they are 'getting their cummuffins' are as bad or worse as having to eat a muffin that has been ejaculated on. More severe and graphic version of come-uppins.