Rabid InuYasha Fan. A term that InuYasha fans use to label themselves proudly.
However, it has also been used by dumbass InuYasha Haters use to degrade InuYasha fans when they don't agree with the fans. The latter is really just an InuYasha version of "Douchebag".
The term originated from the Adult Swim Message Boards to differentiate annoying InuYasha fans from Civilized InuYasha Fans (or C.I.F.s).
I've seen all 167 episodes of InuYasha and own all seven box sets, so that makes me a R.I.F..
Jimmy the RIH: Do you like InuYasha?
Jim the R.I.F.: Yes.
Jimmy the RIH: Well, I hate it, so you suck, you friggin R.I.F.!