hydrogen (n.) Look up hydrogen at Dictionary.com
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.