A fork of Rural Dictionary
It's quite simple actually. It was originally a shortened version for For Your Information. However, people have made up other things that can stand for FYI. For example "Fuck You Idiot". But, that's a bit pointless if you ask me. Saying the whole thing has a grander effect.
(Quoting Shrek 2) Prince Charming: FYI, not my fault! Or.. -FYI!!! -Huh? -Fuck you idiot!!! -What the hell?