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c.1300, "a note, reference," from Old French mencion "mention, memory, speech," from Latin mentionem (nominative mentio) "a calling to mind, a speaking of, a making mention," from root of Old Latin minisci "to think," related to mens (genitive mentis) "mind," from PIE root *men- "think" (see mind (n.)).
1520s, from mention (n.) or else from Middle French mentionner, from Old French mencion. Related: Mentioned; mentioning; mentionable. Don't mention it as a conventional reply to expressions of gratitude or apology is attested from 1840.