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.
To fail to attend friends and loved one's momentous occasions, despite having assured them one would be there, only to instead opt for menial tasks such as smoking pot, gaming, and most possibly masturbation
Matt decided to pull an AD on my birthday this year, the bastard, only to touch himself ever so tenderly while watching Pretty Woman