n. Slap on Forehead. Usually said (on MSN or any form of instant messaging online) when the other person has said something extremely stupid or "uncool". The letter equivalent of the now common =.=". Sof is used for those who are deprived of newer versions of MSN which allows them to send and receive custom emoticons.
A: oh my god, Jesse McCarteney is so hot.
B: sof. you're kidding, no he's not.
An organization of ten homosexual boys originating from WMMHS who rally frequently to convey their extreme affection for one another and refer to themselves as "the society of friends."