ROT13 encryption of the word "sheesh".
"furrfu" came about in the early 1990's used as a response to urban myths (like chain mail floating around) on newsgroup alt.folklore.urban after users on the newsgroup had complaints that the usual response "Sheesh!" was being overused by alot of newbies.
<dude1> I heard that when visiting the cash for clunkers site at cars.gov you have to agree to allowing your computer to become government property!
<dude2> furrfu, thats bs dude, that agreement only applied to dealerships and even then they fixed the agreement to not say that.