"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.