I've heard this idiom several times. But I can't find it in my dictionary.
Some forums in my country said it means "really" or it expresses surprise, and they give me an example:
-I make it!
-You don't say!
-Don't mock me! I‘ll beat him.
But it doesn't fit the context of the samples I've encountered.
A: Mr. Qian, what does he do exactly?
B: Gambling, man! What do you think?
A: Brother! You, me, and him, we're gonna work together.I tell you now. We're gonna work this out.
B: You don't say!
I feel it means "It's obvious. You don't need to tell me that."
Could someone be kind to tell me what exactly this idiom means?