ectomorph

'Ectomorph' traditionally refers to a person who has a fast metabolism and little fat. However, it is more commonly used by those who have little muscle as an excuse. Many skinny people would call themselves 'ectomorphs' or 'hardgainers' but in actual fact they just don't eat enough.

Female: Ewww, why are you so skinny?
Insecure Male: Its because of my genetics. I'm an ectomorph!