Skinny-Fat: When someone is thin and looks great in clothes, but is all flabby underneath. They may have a low weight-to-height ratio, but have a higher than expected body fat level. They have no muscle tone and are weaker than expected.
Often these people eat a horrible diet (which may include high levels of alcohol and/or cigarettes), and use chronic stress to keep their body weight low. This stress may be from their career or in the form of chronic "cardio" exercise. These peoples' physiology are similar or worse than that of a typical obese individual.
Here are some examples: