the.com/parting shot

means A final cutting remark delivered as you're leaving, timed precisely so there's no chance of rebuttal.

from This phrase is a corruption of 'Parthian shot' — the Parthians, ancient horse-archers of Persia, were famous for a battle tactic of feigning retreat, then twisting around in the saddle to fire arrows backward at their pursuers while still galloping away. The image of striking while fleeing fused with the word 'parting,' and by the 1800s the two had merged into the phrase we use now — the military maneuver dissolving into a verbal one.