A shortened version of "Freakin Idiot". You normally say it in a really retarded or high-pitched voice. You can also say it like freh eh! OR fruggin idjiot! It all started because some guy would always say freakin idiot really ugly so his friends started making fun of him. Now he does it on purpose.
I actually don't even know the guy, my friend does. But yeah; just do it. It's fun. :D
"Dude, I think I ran over your dog."
"FRUH UH!! That's my sister!"