1
general::
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 1800s] For a synonym, see YOU DON'T SAY.
American Heritage Idioms
2
general::
Phrase(s): Do tell.
a response to one of a series of statements by another person. (The expression can indicate disinterest. Each word has equal stress. See also You don’t say.) • Bill: The Amazon basin is about ten times the size of France. Mary: Do tell.
McGrawhill's American Idioms And Phrasal Verbs