FAT is short for "Fuck All That".
These three short simple and to the point words conveniently shortened and combined together to form a three-letter acronym or initialism, are a force to be reckoned with. If anyone ever says FAT to you, you'll get the message, crystal clear...if not you'll maybe hit the gym up for some cardio.
she says, "wait honey you can't do that. you can't leave! there's people coming here, your parents, neighbors, the kids soccer coach..."
he responds, "FAT!! I'm out!" and leaves to enjoy his manliness to get a burger and drink beers with the boys, avoiding all the drama.