Third person singular present tense of fornicate; engages in sexual relations outside of marriage.
From fornicate + -s (third person singular marker). As English formalized verb conjugations, this form became the standard way to describe what one person does in the present tense.
In Shakespeare's plays, you'll see 'fornicates' used to describe morally questionable characters—it was a serious accusation with legal consequences, not just a casual description.
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