Meaning

noun extramarital sex that willfully and maliciously interferes with marriage relations

originWith change of suffix, from the Old French adultere (“violation of conjugal faith”) (in Bernard of Clairvaux, Sermons, 12c.), a learnèd borrowing from Latin adulterium, from adulterō. Replaced the older form avoutrie, from the inherited Old French forms avouterie or aoulterie, while keeping its suffix. Compare French adultère (“adultery”). Displaced Old English ǣwbryċe. Not related to adult.

Etymology from Wiktionary, CC BY-SA 3.0