ectomorph

This term originates from somatotyping, a system used to categorize human physiques. It carries a clinical or biological tone and is frequently used in fitness, bodybuilding, and medical contexts to describe a specific genetic predisposition toward leanness. While generally neutral, it can be used descriptively to explain why certain individuals struggle to gain muscle mass.

When used as an adjective, it describes the quality of the build itself. When used as a noun, it refers to the person possessing that build. It is distinct from terms like "skinny" or "slender," which are more general descriptors; "ectomorph" implies an underlying biological or constitutional type.