you don't say!

you don't say!
you don't say! (informal)
An astonished rejoinder, or ironic response to the obvious or unamusing
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Main Entry:say

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informal used to express amazement or disbelief

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you don't ˈsay! idiom
(informal, often ironic) used to express surprise

‘They left without us.’ ‘You don't say!’

(= I'm not surprised)
Main entry:sayidiom

Useful english dictionary. 2012.