OK, Mine is pretty simple. My name is Mike Trump. I have been known as Trumpy since I started school in the 70's. It stuck and there you go. It is a convention here in NZ that if you have a one syllablle((sp??) surname, you instantly get a y at the end of it. It is an affectionate name. I have however been referred to throughout my Fire Service career as <Insert Rank> Trump. Not unlike the military.
"Mine came from my motocross days in the early 1990's."
So I guess MXSlick already knows what WFO stands for. For the rest of you, it stands for "Wide Freaking Open" (cleaned up slightly for public consumption) and referred to somebody whose throttle hand was stronger than his brain.