Fornication
Sexual intercourse that is "illicit", outside of marriage. The term "fornication" has close medical relatives by the name of "fornix", the word for "arch" in Latin. In anatomy there is the "fornix conjunctivae" (the fornix of the conjunctiva of the eye) and the "fornix uteri or vaginae" (the fornix of the uterus or vagina). 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": fornication.
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for·ni·ca·tion .fȯr-nə-'kā-shən n consensual sexual intercourse between two persons not married to each other
Medical dictionary.
2011.