from The American Heritage® Dictionary of the English Language, 4th Edition
- n. 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.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. An insult or barbed comment issued as the speaker departs or the conversation comes to an end.
Believed to be a variation of Parthian shot, referring to a military maneuver in which Parthian troops retreating on horseback turned back to face the enemy and shot while retreating. (Wiktionary)