When the body is proportioned wrong. When the upper half (waist up) is much larger than it should be for the bottom half (waist down.) The upper half has lots of rolls, while the legs are toned. It can also be vise versa, the legs are much fatter then the upper half, very slim waist line and toned abs with thunder thighs.
"whoaaa that girl is weird looking, she is most def a skinny fat."
"that girls legs are so skinny.. but her upper half is so big. must be a skinny fat."