[piasa]

The Parthian shot is a light horse military tactic made famous by the Parthians, an ancient Iranian people. While being in real or feigned retreat, their horse archers would turn their bodies back in full gallop to shoot at the pursuing enemy.

Over time the spelling and pronounciation was corrupted until it became "parting shot."

[chajin]

Learned something today; as the Parthian would say in modern times, muchakeram (thank you)