1550s, "have illicit sexual intercourse" (said of an unmarried person), from Late Latin fornicatus, past participle of fornicari "to fornicate," from Latin fornix (genitive fornicis) "brothel" (Juvenal, Horace), originally "arch, vaulted chamber, a vaulted opening, a covered way," probably an extension, based on appearance, from a source akin to fornus "brick oven of arched or domed shape" (from PIE root *gwher- "to heat, warm"). Perhaps in some cases a back-formation from fornication. Related: Fornicated; fornicating.
