A person who is not overweight and have skinny look but still have a high fat percentage and low muscular mass. Usually those people have a low caloric diet, that's why they are skinny, but are not involved in any sports activities or trainings and that's why they don’t have any muscle. Since between the bone and the skin those people only have fat, the skin can be deformed easily because the skin layer is located on an unstable matter (fat).
Skinny fat woman look sexy from a certain distance but they have soft jelly skin and still have some cellulite even if they are in there early 20.
The Antonym of ‘Skinny fat’ could be ‘Fit’ or ‘Tight’ or ‘Athletic’.