Adultery
late 15th century: from the obsolete noun adulter, from Latin adulter ‘adulterer’, replacing an earlier form avoutrie, from Old French avouterie .
wiktionary
From the Old French scholarly form adultere(“violation of conjugal faith”) (in Bernard of Clairvaux, Sermons, 12c.), from Latin adulterium, from adulter. Replaced the older form avoutrie, from the popular Old French forms avouterie or aoulterie. Compare French adultère(“adultery”). Displaced Old English ǣwbryċe. Not related to adult.
etymonline
adultery (n.)
"voluntary violation of the marriage bed," c. 1300, avoutrie, from Old French avouterie (12c., later adulterie, Modern French adultĕre), noun of condition from avoutre, from Latin adulterare "commit adultery; corrupt," from ad "to" (see ad-) + alterare "to alter" (see alter). Compare adulteration. The spelling was corrected toward Latin from early 15c. in English, following French (see ad-).
In Middle English, also "sex between husband and wife for recreational purposes; idolatry, perversion, heresy." As a crime, formerly classified as single adultery (with an unmarried person) and double adultery (with a married person). The Old English word was æwbryce "breach of law(ful marriage)" (similar formation in German Ehebruch). In translations of the 7th Commandment it is understood to mean "lewdness or unchastity" of any kind, in act or thought.
