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 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]

The last word. At the end of a heated discussion or argument, you unleash a zinger of a remark as you leave. You've just made a parting shot. The phrase is very often said as “Parthian shot.” The Parthians were a Persian tribe that developed the cavalry tactic of retreating in order to draw their enemy after them, whereupon they would turn in the saddle and fire a barrage of arrows. Although some scholars say “parting” came from “Parthian,” others say it's coincidental.