Accusation delivered to silent person that is so obvious! (3,4,3)

I believe the answer is:

you dont say

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But I can't explain it at all! Could you help me understand?

(Other definitions for you dont say that I've seen before include "Surprised comment" , "How astonishing,'" , "No kidding!" , "Ironic response to an obvious remark" , "one thinks" .)

I've seen this clue in The Guardian.

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