late 15c., "caustic, biting, severe" (of words, speech), from Old French mordant, literally "biting," present participle of mordre "to bite," from Latin mordēre "to bite, bite into; nip, sting;" figuratively "to pain, cause hurt," which is perhaps from an extended form of PIE root *mer- "to rub away, harm." Related: Mordantly.
The noun is first attested in a now-obsolete or archaic sense of "ornamented hooked clasp of a belt or girdle" (mid-14c.), from Old French mordant in this sense. In dyeing, "substance used in fixing colors," it is attested by 1791; as an adjective in dyeing, "having the property of fixing colors," by 1902. Related: Mordancy; mordantly.