People who doesn't look fat or overweight from the outside (no 'fat folds') and still are considered 'fat' when they are submitted to hi-tech body-scanning techniques (like MRIs), to discover that they are what doctors call 'centrally-obese': having large fat-cells in their vital organs and subcutaneous skin deposits which is high risk for heart disease and can double risk of CDV (cardiovascular disease) in men.
"She ate a double-quarter pounder meal from McDonalds, 3 pieces of chicken from KFC, and some yogurt from TCBY....all in one sitting"
"Damn she THICKE, SKINNYFAT!!!!"
Also known as, Skinny on the outside, fat and unhealthy on the inside.
B: "Well, she looked good at the bar, but when I got her clothes off, she was totally skinny-fat."