To engage in sexual relations outside of marriage; a formal or religious term for sexual intercourse between unmarried people.
From Latin 'fornicatus,' derived from 'fornix' meaning vault or arch, which was slang in Rome for the arches under streets where prostitutes worked.
The etymology is delightfully architectural—Roman prostitutes literally worked in the arches (fornix) under buildings, so their location became the Latin slang for the act itself, which eventually became the formal word we use today.
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