Crummy = Creepy and Funny.
When someone makes a joke that is both HILARIOUS but also somewhat unerving and you're not really sure how to feel.
This is that feeling.
Originated from cyber English, with intergrated french, youth speak. Some may question the change from 'n's to 'm's. This is merely a progression of popularly used languge, as are irregular conjugation of commonly used verbs (e.g. I don't be but I am). Phonetically, english speakers are more accustom to the 'm' sound so forming the sound is more
comfortable hence it's linguistically accepted development.
It also happens to be a
double entendre to a bad a joke. This only adds to it's perfect meaning.