1940, from meso- + Greek morphe "form" (see Morpheus). Coined by W.H. Shelton; the reference is to the mesodermal layer of the embryo. Related: Mesomorphic (attested from 1923 in chemistry).
mesomorph mes·o·morph (měz'ə-môrf', měs'-)n. An individual characterized by a robust muscular body build in which tissues derived from the embryonic mesodermal layer predominate.