mesomorph n.

"person with a powerful, compact body build," 1940, from mesoderm + -morph, from Greek morphe "form, shape; beauty, outward appearance," a word of uncertain etymology. Coined by American psychologist William H. Sheldon (1898-1977); the reference is to the mesodermal layer of the embryo, from which physical structures develop. Related: Mesomorphic (attested from 1923 in chemistry, a separate coinage in reference to a state of a liquid crystal).