Abbreviation for "You're Not My Friend!". Often saved for folks with whom we are quarreling, joking, or otherwise truly not friends.
Person 1: "Dude, shut up."
Person 2: "You can't tell me to shut up! YNMF!"
Short for "your not my friend".
Something to be said (preferably shouted) while losing an argument. So as to say, "fine you may win this argument, but as a fucktard, i will disown friendship of you"
Person 1: blah, blah, <rational argument>.
Person 2 (in desperation): YNMF!