Merriam-Webster Collegiate Dictionary - vitriol

 noun  Etymology: Middle English, from Anglo-French ~e, from Medieval Latin ~um, alteration of Late Latin vitreolum, neuter of vitreolus glassy, from Latin vitreus vitreous  Date: 14th century  1.  a. a sulfate of any of various metals (as copper, iron, or zinc); especially a glassy hydrate of such a sulfate  b. oil of ~  2. something felt to resemble ~ especially in caustic quality; especially virulence of feeling or of speech  • ~ic adjective