Acronym for "You're a fuckin' douchebag!"
Most properly used when texting or posting on facebook, or perhaps verbalized as "Y-A-F-D" when kids are around or in an extremely "proper" situation.
Jim: "Hey fb ppl, I was driving down the street after the big rain storm and came across a huge puddle. Went through it and splashed this old lady standin' on the corner! Ha ha, you shoulda seen her face!"
Billy: "Dude, you think that's funny? YAFD!"