Short for Fat Ass Nerd, to be said in MMORPGs like WoW & Runescape. The typical F.A.N. is a super high level player that thinks he is superior to all other players because of his online persona. Often, he will shout out random, condescending phrases like, "You noob!" and brag about his stats or fast internet speed. Or make fun of yours. The typical F.A.N. has such a high level character only because he sits on his ass all day long playing the damn game, eating Twinkies by the boxful and chugging 2 liters of Mountain Dew. Don't feel bad when a F.A.N. insults you; he's only trying to make up for the fact that he's an utter failure in real life.
See also virgin
"You noob! You should hop worlds because I've been here fighting these Greater Demons for the past 16 hours, and my needs are clearly more important than yours!"
"Shut up, F.A.N., this is the internet and everyone is equal here...and you should be grateful for that, because you suck in real life."