MFX

Abbreviation for motherfucking EXTREME
When extreme simply isn't enough to describe the intensity/coolness of a person or situation, one may use MFX to really sell just how extreme it is

cool guy: does a triple backflip while playing guitar on fire then wrestles a shark
friend 1: holy frick that guy is extreme
friend 2: nah bro that guy is MFX
friend 1: oh for sure