late 14c., "sulphate of iron," from Old French vitriol (13c.), from Medieval Latin vitriolum "vitriol," noun use of neuter of vitriolus, variant of Late Latin vitreolus "of glass," from Latin vitreus "of glass, glassy," from vitrum "glass" (see vitreous). So called from its glassy appearance in certain states. Meaning "bitter or caustic feelings" first attested 1769, in reference to the corrosive properties of vitriol (when heated it produces sulfuric acid, formerly called oil of vitriol).
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