Fornicatio, -onis

etymologically, fornication derives from the same Latin root as oven: namely, a fornix, -icis is simply a vault. As stone ovens were vaulted, an oven was a fornax, -acis, from which the Italian forno="oven" is derived, or the english furnace.

Also vaulted were cellars ("vaulted"=fornicatus), and cellars were notoriously the locations of Roman brothels, the passive verb fornico, -or came to mean "to whore"; and fornicatio, -onis was the ambiguously corresponding abstract noun which later became the english fornication.

When in Rome, do as the Romans do.