You don't say is a dry reply to news that is not surprising at all. Oh, you don't say.
It is sarcasm dressed as astonishment, and the tone does all the work. Said sincerely, which is rare, it means genuine surprise.
1phraseyou don't sayA dry reply to news that is not surprising at all.
“When he said the trains were late, she said you don't say.”
you don't say is a phrase.
Practised in Small Talk & Everyday Phrases