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).
Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or from cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames. In chemistry, indicating a higher valence than names in -ous (first in benzoic, 1791).
In Middle English and after often spelled -ick, -ike, -ique. Variant forms in -ick (critick, ethick) were common in early Modern English and survived in English dictionaries into early 19c. This spelling was supported by Johnson but opposed by Webster, who prevailed.