How come "talk about" can be used to emphasize something? What's the etymology of that?
I assume you're referring to expressions like "Talk about stupid! He can't even count a dozen eggs and get 12."

I don't know if you're going to have much success finding the etymology of that expression. It seems to me that it is simply a short form of "If you want to talk about X, listen to this!", in which "talk about" has its literal meaning, that is, is not used as a special idiom.

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Thank you, Jim!

So, you mean when we use "talk about" to that effect, it has to be coupled with the "listen to this" part? In other words, we cannot use the "Talk about stuid!" part alone?
The "listen" part was just a suggestive paraphrase for the use of "talk about" before the story which serves as the illustration of what we are talking about; we don't actually say any more than "Talk about X!"

"Talk about stupid!" could be used in these situations:

To introduce an illustration of stupidity.
To conclude an illustration of stupidity.
To comment on an illustration of stupidity made by someone else.

The paraphrase will vary slightly depending on the usage.

"Talk about X!" is like "How X!", where X is an adjective. "talk about" belongs squarely in the world of informal conversation, of course.

So the illustration part is the listing part, from a certain point of view, right?

Thanks, Jim!
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