How surprising, is that true? Also, I find that hard to believe. For example, I've been working on this project for two years.—You don't say, or The man who runs this soup kitchen is a real saint.—You don't say! This expression, a shortening of you don't say so, may be used straightforwardly or ironically. [Late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
How to use you don't say in a sentence
Joffre stopped and turned with a "you-don't-say-so" gesture and a toss of his head at something that Castelnau had told him.My Second Year of the War | Frederick Palmer