I've never given any thought to the phrase parting shot - I guess I just assumed that it was a shot made right before you depart - so I was shocked to learn that it's actually a folk etymologization of the phrase Parthian shot! That term referred to a particularly difficult military tactic used by the Parthinian people of Ancient Persia wherein retreating archers on horseback would turn around on the horse and shoot behind them. This came to be a metaphor in English around the nineteenth century, and was corrupted into the modern form relatively quickly. Parthinian is the endonym for Parthia, which comes from the Old Persian root p-r-th-n, which also referred to the general region. According to Google NGrams, literary usage of parting shot peaked in 2015 and has been declining since.
Adam Aleksic is a sophomore studying linguistics and government at Harvard University. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.