Did you not hear me?

One definition of an idiom: a phrase or sequence of words that conveys a meaning that cannot be explained by its construction.

Sometimes the construction isn't grammatical or doesn't occur elsewhere in a speaker's language (e.g. a couple three __s or you bet you). Most commonly cited idioms have a grammatical construction but a meaning that can't be extracted other than by convention (e.g. she had to eat crow).

Such idioms are often picturesque and many have traceable metaphorical meanings.

To trip the light fantastic

Filling your eyes before your stomach

Lay an egg

Shoot from the hip

Pulling your leg

But for the moment I'm more interested in those utterly prosaic idioms that don't paint a picture or rely on an image or sound very colorful. The idioms that sometimes don't even sound like idioms.

I've been wondering for a while about the phrase You don't say. In a 1935 article in Language L.W. Merryweather uses it as the translation for the phrase "The hell you say!"¹ Allen Walker Read includes it in a fuller form ("You don't say so!") alongside a quote illustrating old scratch as a 1848 "Nantucketism."²

It can express interest with little or no surprise.

• That new restaurant was quite good.
  
• You don't say. We'll have to try it.

It can express surprise.

• I won the lottery!
  
• You don't say! That's great.

It can be an ironic expression of surprise in response to an obvious statement.

• The sun will rise tomorrow.
  
• You don't say. And here I thought the government cut that program.

But the path to its meaning isn't clear. There could easily be some connection between disbelief that a claim is true and disbelief that a claim has been--or should be--uttered. Somewhat as 'don't tell me' connects I don't want to hear that with I don't want that to be true.

So -- you don't say (so) [unless it's true]?

It's an inelegant explanation. I'm entertaining better offers. I know they're out there.

[Update:
Let me take this opportunity to encourage you to read John McIntyre's blog: You Don't Say.]
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1. American Speech, Vol. 6, No. 6 (Aug., 1931), p. 433
2. American Speech, Vol. 10, No. 1 (Feb., 1935), p. 41