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When someone is clinically overweight but still has a good body shape or able to run faster or gym harder than a slimmer person.

See also: Chubby Fit
I thought he was muscled but when he took his shirt off I realized he is just fat fit

Ryan: Dude did you see that guy on the track. He totally out ran me but he is fat.
Steve: Yeah but he is fat fit.
by -T- September 24, 2014
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