You don't say

You don't say. 

1. Inf. a general response to something that someone has said. (Expresses a little polite surprise or interest, but not disbelief.) Bill: I'm starting work on a new job next Monday. Bob: You don't say. Sally: The Jones boys are keeping a pet snake. Alice: You don't say.

2. . Inf. You have just said something that everybody already knows. Bill: I think I'm beginning to put on a little weight. Jane: You don't say. John: My goodness, prices are getting high. Sue: You don't say.

See also: say