A fork of Rural Dictionary
Stands for "Fuck you, fatty!" based on an Internet discussion wherein a self-righteous workoutaholic pseudonutritionist was passive-aggressively tisk-tisking about a perfectly healthy-sized (i.e., not anorexic) woman eating a cookie. Commenters who saw right through the kidding/not kidding dubbed her attitude "FUF". The rest is history.
I am not going to put up with this nonsense because FUF, that's why!