Fornication
Sexual intercourse that is 'illicit', outside of marriage.
The term 'fornication' has a close medical relative by the name of 'fornix', the word for 'arch' in Latin.
It seems that the Roman prostitutes used to hang out under the arches of certain public buildings.
The act of carrying on an illicit relationship consequently came to be called 'going under the arches' or fornication.
Not to be confused with formication.