1933, coined by U.S. chemist Harold C. Urey, with Modern Latin ending + Greek deuterion, neuter of deuterios "having second place," from deuteros "next, second," according to some sources from duo (see two), but according to Watkins the ground sense is "missing" and the Greek word is from PIE from *deu-tero-, suffixed form of root *deu- "to lack, be wanting." So called because it is twice the mass of hydrogen.