ectomorph

noun2

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. — 1990
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.