fornication: [13] Latin fornix denoted an ‘arch’ or ‘vault’, and hence came to be used in the late republican period for the sort of vaulted underground dwellings where the dregs of Roman society – tramps, prostitutes, petty criminals, etc – lived. Early Christian writers homed in on the prostitutes, and employed the term with the specific meaning ‘brothel’, whence the verb fornicārī ‘have illicit sexual intercourse’ and its derivative fornicatiō, source of English fornication.
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" (see furnace). 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.
