early 15c., dissonaunt, "at variance, disagreeing," from Old French dissonant (13c.) and directly from Latin dissonantem (nominative dissonans), present participle of dissonare "differ in sound," from dis- "apart" (see dis-) + sonare "to sound, make a noise," "to sound," from PIE *swene-, from root *swen- "to sound." Meaning "discordant in sound, harsh" is from 1570s. Related: Dissonantly.
