Incorrect spelling of F OFF (fuck off). Often used in MSN Messenger where it is associated with an emoticon that shows an angry face sticking its middle finger up at the user. This emoticon is often mis-interpreted as an unoffensive emoticon involving a man with his dick out.
MSN Nerd 1: Guess what: UR FAT
MSN Nerd 2: *foof*
MSN Nerd 1: foof!!! lol!!! ur such a n00b!
MSN Hater: You twats, why don't youget a life
When I was young a Foof was a politer way of saying Fart, usually used by children or by adults when speaking to children. By the way, I'm Welsh, so we would probably have spelled it Fwf :)