UnFuckingBelievable. Something so completely off the wall, out of the ordinary, uncommon, or otherwise difficult to comprehend as a fact or part of one's perception of reality, that it defies belief.
Dude, I was driving along minding my own business when a cop pulls me over. He walks up to my car all slow and sanctimonious like they always do and he leans down towards the window and says to me 'Hey, I'll forget about your speeding infraction if you can score me some kind bud.'
OMFG! That is so UFB!