a parting shot
A final, usually critical remark made specifically to have an impact on the listener(s). He couldn't resist getting a parting shot in at the hiring manager as he stormed out of her office.
a ˌparting ˈshot(also a ˌParthian ˈshot) a remark or action, often an unkind one, that somebody makes just as they are leaving: As Jim walked out of the door, his parting shot was, ‘I never want to see any of you again.’Parthia was a kingdom in ancient times. The Parthians used to fire arrows at the enemy as they were retreating from battle.
parting shot, a
A final insult hurled as one is leaving, or the last word in an argument. Most authorities agree that this term is a distortion of Parthian shot, alluding to the ancient Parthians (first century b.c.), renowned archers and horsemen, who would turn in flight to discharge arrows at the pursuing enemy. There is no firm evidence for this etymology, but it seems reasonable. John Galsworthy used the term in A Man of Property (1906): “He could not resist a parting shot, ‘H’mmm! All flourishing at home? Any little Soameses yet?’”
See also: parting