A phrase used to express surprise about something, as in Jane's getting married again? Do tell. This expression does not necessarily ask the speaker to provide more details but merely expresses one's astonishment. [Colloquial; first half of 1800] For a synonym, see you don't say.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.