RUAFI is an acronym for "aRe yoU A Fucking Idiot?"
Often the usage is rhetorical.
This can also be used as a statement of fact, rather than a question, by changing the order of the first two letters as in:
URAFI ("yoU aRe A Fucking Idiot")
This phrase is sometimes used on neowin.net to avoid the swear word filter and to skim the radar of the moderators who look for personal attacks.
xboxfan: I wonder how many PS3s it would take to heat my home?