Short for defective. Can be used for a multitude of things, from situations that have gone wrong, to things that have broken, to an ugly girls face.
"Dude, you straight deefed that bowl"
"Dude, I was going to smash, but that chicks face was straight deefed."
"Dude, if I fart now, I'm straight going to deef my pants"
1. That of which is deeferent or different. Often times meaning strange or weird.
2. One who claims themselves to be different, individual, unique, or nonconforming. Deefs are usually marked by black clothing, tight clothing, dickie pants cut into shorts, converse shoes, and/or patches on one's backpack.
All of those deef kids sit on the hill with the patches on their backpack and dickie cut off shorts.