This term is used when a person is thin, yet does not eat healthy or take care of themselves. They think they don't need to watch the junk they eat because they are thin. They can be more unhealthy then an overweight person.
Also known as, Skinny on the outside, fat and unhealthy on the inside.
If a person can eat pizza, burgers, candy all day long and not gain weight, they would be considered skinny fat. They can still die from the same weight related diseases. That is Skinny-Fat