Acronym for Me First, Fuck You.
MFFY is used to describe a person who will happily inconvenience another person in order to convenience themselves. Originated in internet driving forums.
Rob: Hey Barry, why are you so angry?
Barry: Some MFFY cut me off on the way here. He flew up behind me just before his turn off, decided to pass me then moved back in front of me and slowed right down to make his turn.