a shorten version of
the word "MO", which is a shortened version of the phrase "MO-FO", which is also shortened for the term Mother Fucker.
Basically it's a quick and easy way to call
someone a mother fucker without having to use all those syllables.
"You are such an MMF!"
Jason: Hey guys! wow you look like
crap today, well i gotta
run BYE!
Todd: What is that guy's problem?
Gilroy: I don't know
dude, i guess he is just an mmf