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.