parting shot definition by American Heritage Dictionary
nounThe American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
An act of aggression or retaliation, such as a retort or threat, that is made upon one's departure or at the end of a heated discussion.
parting shot - Phrases/Idioms
A final insult or last word in an argument, as in As she stalked out, Jane hurled as a parting shot, “And I quit!” This idiom apparently originated as a corruption of Parthian shot, referring to the practice of ancient Parthian warriors of turning back to shoot at their pursuers. [Late 1800s]