1. To engage in any form of illicit sexual intercourse, as between a spouse and an unmarried person or between two married persons who are not married to each other.
2. To have sexual intercourse. Humorously defined by Frederic Mullally in The Penthouse Sexicon (1968): ' Nine-letter euphemism for four-letter euphoria.' See copulation for synonyms.
3. To engage in indiscrimitate and/or promiscuoussex. Etymology: First recorded in Middle English about 1300. From the Latin fornax , meaning oven, kiln or fumace, and from fornix, meaning arch, vault or vaulted room, from the shape of Roman ovens, later the arched underground vaults of Rome, and finally a brothel, because the Roman vaults were used by poor, unskilled prostitutes.