you don't say字面意思是你别说话，但真正的意思是：是真的吗；真想不到；不见得吧。

【例句】
You don't say. What kind of book are you writing?
真的吗，你在写什么样的书呢？

You don't say. I hope you will really help them then.
真的吗，我希望你到时候真的会帮助他们。