Q: After years of using the phrase “not to mention,” I wonder if I should be writing “not to forget” instead. It seems illogical to use “not to mention” to introduce something that you’re about to mention, as in this sentence: “My favorite desserts are apple pie and cheesecake, not to mention chocolate ice cream.”
A: The phrase you mention (“not to mention”) is an idiomatic expression in English similar to another one, “let alone.” What the speaker means is that the thing that follows, whatever it is, hardly needs to be mentioned, or is to be taken almost for granted, or is so obvious that it’s practically unnecessary to say it.
Examples: “She has three dogs and five cats, not to mention the goldfish.” Or: “We’ve never been to Canada or Mexico, let alone Asia.”
It’s true, as you say, that the thing not to be mentioned is in fact mentioned, just as the thing to be let alone is NOT let alone; it is in fact very relevant. Such are the oddities of idiom!
Although “not to mention” may not make literal sense, it’s been in common use in English since at least the 17th century. The earliest published reference in the Oxford English Dictionary comes from Milton’s 1644 treatise Of Education: “Not to mention the learned correspondence which you hold in forreigne parts ….”
We were asked not long ago about another idiomatic expression, “it’s not for nothing that” (meaning “it’s significant that” or “there’s a reason that”). We found many citations for this usage in the OED, including this one from Shakespeare’s Merchant of Venice: “It was not for nothing that my nose fell a bleeding on blacke monday last.”
Speakers (or writers) use expressions like these – “not to mention,” “let alone,” “not for nothing,” and so on – as signals to clue the listener/reader in to the greater (or lesser) significance of what follows.
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