Abreviation of "What the motherfucking fuck?" Used to express intense confusion, exasperation, or misunderstanding. Can also be stated as "WTMFSQUARED."
"Hey man, I brought you this dead cat carcass."
"WTMFF, dude?! Throw that shit away!"
"I'm not gonna be able to get you that half today, dog."
"WTMFSQUARED, man? You promised."