An exclamation of surprise, incredulity, etc: You shot the dog? Do tell!(1840+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with do tell
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.
[ ; first half of 1800