An abbreviation for "why the fuck not?". Used by a person when they are intrigued as to why the fuck someone would not agree with something, want to do something, have done something, etc. Its history traces itself back to a windows live conversation.
Person #1: I don't want to go to a lounge
Person #2: YTFN
Person #1: I just want to lounge, no shape-up, chilling, same Tee i had on for a week, what i got on it reeks.
Person #2: you nasty