Used to introduce an additional point that strengthens or emphasizes what has already been said.
This phrase developed in the 17th century as a rhetorical device, literally meaning 'I won't even mention this other thing' while actually mentioning it. It's a form of praeteritio, a classical rhetorical figure where you bring attention to something by claiming you won't discuss it.
The phrase creates a delicious irony - by saying 'not to mention,' you're actually mentioning the very thing you claim not to mention. It's become such a common intensifier that speakers often use it unconsciously, making it one of English's most paradoxical everyday expressions.
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