Talking about something briefly or casually; or referring to something.
From Old French 'mentir' (to lie), but mentioning comes from 'mention' which entered English from Old French, from Latin 'mentio' (a calling to mind). The etymology is confused, but the modern sense is 'to bring to mind.'
The phrase 'not to mention' is weird because it literally means you're NOT supposed to mention something—but then you do. It's a rhetorical trick that's so common we don't even notice the logical contradiction anymore.
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