Usually means Fat Ass Fucker, normally used in Internet Relay Chat Networks suchs as VidGameChat.Net.
Sheik is a faf.
Nizzo is a faf.
Huntr is the biggest faf there is.
twEAkz fafs me up.
CaBiaR is a complete faf.
Yo, tm2, you faf.
You faf, what's up?
Instead of fafing around, I'll get an expert to fix it