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1777, "phosphorescent," from phosphorus + -ous. The chemical sense (1794) is immediately from French phosphoreux.
phosphorous phos·pho·rous (fŏs'fər-əs, fŏs-fôr'əs)adj. Of, relating to, or containing phosphorus, especially with a valence of 3 or a valence lower than that of a comparable phosphoric compound.