c. 1300, from Old French fornicacion "fornication, lewdness; prostitution; idolatry" (12c.), from Late Latin fornicationem (nominative fornicatio), noun of action from past-participle stem 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"). Strictly, "voluntary sex between an unmarried man and an unmarried woman;" extended in the Bible to adultery. The sense extension in Latin is perhaps because Roman prostitutes commonly solicited from under the arches of certain buildings.