An acronym for "Temporary Freedom Week/Weekend". Usually implying that your wife/hubby/kids/girlfriend are away for a period of time and you are free to engage in shenanigans.
Guy1 : "What are you up to this weekend?"
Guy2 : "I've got a little TFW going on. Wanna pick up some midgets and get a group orgy going?"
Guy1 : "Those sound like some shenanigans that I would be down for."