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c.1600, from Latin nitrosus, from nitrum (see nitre). Originally "of nitre, pertaining to nitre;" more precise use in chemistry (designating a compound in which the nitrogen has a lower valence than the corresponding nitric compound) is from 1780s. Nitrous oxide attested from 1800.
nitrous ni·trous (nī'trəs)
Of, derived from, or containing nitrogen, especially in a valence state lower than that in a comparable nitric compound.
Containing nitrogen, especially nitrogen with a valence of 3. Compare nitric.