ectomorph

noun

1. Someone with a lean, only slightly muscular body

   “Also there was a woman I hadn't met, a chain-smoking French ectomorph whose job it was to make sense of records of public spending in the Ministry of Finance.”

2. Theoretical body type in which a person has a high metabolism. Such a person can easily maintain a low fat physique, but does not add muscle or body weight easily.

   “The ectomorph body type is characterized by narrow shoulders and hips and a lean build. Its naturally fast metabolism makes it easy for ectomorphs to lose or maintain body fat levels but more difficult to build muscle.”