word-forming element generally meaning "outside, external," before vowels ect-, from Latinized form of Greek adverb ektos "outside, out of; free from; exempt" (opposed to entos), used to form compounds in Greek (such as ektomē "a cutting out"); related to Greek ek, ex "out," from PIE *eghs "out" (see ex-).
"person with a slim physique and an introverted, thoughtful personality," 1940, coined by American psychologist William H. Sheldon from ectoderm + -morph, from Greek morphē "form, shape; beauty, outward appearance," a word of uncertain etymology. So called for the embryonic ectoderm, which develops the nervous system. Related: Ectomorphic.
<a href="https://www.etymonline.com/word/ectoderm">Etymology of ectoderm by etymonline</a>
Harper, D. (n.d.). Etymology of ectoderm. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/ectoderm