hydrogen (n.)

colorless, gaseous element, 1791, hydrogene, from French hydrogène (Modern Latin hydrogenium), coined 1787 by G. de Morveau, Lavoisier, Berthollet, and Fourcroy from Greek hydr-, stem of hydor "water" (from suffixed form of PIE root *wed- (1) "water; wet") + French -gène "producing" (see -gen).

So called because it forms water when exposed to oxygen. Nativized in Russian as vodorod; in German, it is wasserstoff, "water-stuff." An earlier name for it in English was Cavendish's inflammable air (1767). Hydrogen bomb first recorded 1947; shortened form H-bomb is from 1950.