Fornication, the rather whimsical-sounding word for intercourse between unmarried people, has mostly been replaced by the phrase extramarital sex nowadays because of increased association with, and parodies of, right-wing hyper-religiosity. However, back when it was first used in a fourteenth century poem, it was in a far more serious context. The word, which has also been spelled fornicacion, fornycacyoun, fornicacioun, and fornycacion throughout the ages, traces to (through Old French) the Latin word fornix, meaning "brothel" - the connection being that a lot of fornication took place in brothels. Since many Roman brothels were in vaulted chambers, that may have meant "arch" earlier on, and the term has cognates hinting that it may be from a Proto-Indo-European root meaning "to warm", but etymologists aren't absolutely sure.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd