late 14c., "sulphate of iron," from O.Fr. vitriol (13c.), from M.L. vitriolum "vitriol," from neut. of vitriolus, from L.L. vitreolus "of glass," from L. vitreus "of glass, glassy," from vitrium "glass" (see vitreous). So called from its glassy appearance in certain states. Meaning "bitter or caustic feelings" first attested 1769, in allusion to the corrosive properties of vitriol.