TATSNBN is The Abomination That Shall Not Be Named A Term used to refer to the 4th edition of the Dungeons & Dragons game (produced) by Wizards of the Coast, used primarily by those who prefer the earlier (TSR-produced) versions. It is a tongue in cheek play on the abbreviation TETSNBN.
something you say to people when you don't know what to say, you don't like them, they are really wildin, or you just weren't paying attention. It is the perfect response every time.