late 15c., "caustic" (of words, speech), from Middle French mordant, literally "biting," present participle of mordre "to bite," from Latin mordere "to bite, bite into; nip, sting;" figuratively "to pain, cause hurt," perhaps from PIE root mer- (2) "to rub away, harm" (see smart, v.). Related: Mordantly. The noun sense in dyeing is first recorded 1791; the adjective in this sense is from 1902. Related: Mordancy; mordantly.