parting shot

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.

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]