What does 'you don't say' mean?

What it means (usually): Sarcastic surprise — meaning "that's completely obvious" or "did you really need to tell me that?" The literal meaning ("really? you're saying that?") has been almost entirely replaced by irony.

"It's raining outside." "You don't say." = I can see that. Obviously.

The rare genuine use: In older English or very polite contexts, "you don't say" can express genuine mild surprise. "She got the role!" "You don't say!" But this sounds dated.

How to read it: In modern English, assume sarcasm unless the context strongly suggests genuine surprise. The sarcastic tone is usually flat or slightly exaggerated.

• "Really?" — neutral
• "No way!" — casual disbelief
• "Is that right?" — mildly surprised, quite British

Register: Casual. The sarcastic version is common in ironic British and American conversation.

Tags: sarcasm, idioms, counterintuitive phrases, social English