A thin person who exhibits traits that are normally found exclusively in fatties, however due to compulsive exercise or a fast metabolic system is not one him/herself.
Such traits include but are not limited to:
Eating like there's no tomorrow
Sucking dick/pussy better than a Hoover
Fucking for validation
Wearing party hats
Matt eats 6 supersized junk food meals a day, he is such a closet fatty.