A final remark, usually cutting or derogatory, made just before departing.
A 'parting shot' is now a metaphorical term but it clearly alludes to the shooting of weapons. The first such literal reference that I've come across is in the writings of John McLeod, who was surgeon of His Majesty's Ship Alceste. McLeod includes this comment in A Narrative of a Voyage to The Yellow Sea, 1818:
The consort, firing a parting shot, bore up round the north end of the island, and escaped.
The figurative use of the phrase comes not much later, in the records of the Religious Society of Friends (The Quakers) - The Friend or, Advocate of Truth, 1828:
I think it would be much more becoming..., if you could separate without giving each other a parting shot. If you could but use this short sentence, "we cannot agree and therefore we separate."
That derivation of 'parting shot' appears to be very simple and straightforward. Not so fast; enter the Parthians and their 'Parthian shots'. The Partians were an ancient race who lived in north-east Persia. They were renowned archers and horsemen and were known for their practice of confusing the enemy by pretending to flee and firing arrows backwards while retreating - not the easiest thing to do on a galloping horse. The tactic must have been successful as in first century B.C. Parthia stretched from the Euphrates to the Indus rivers, covering most of what is now Iran, Iraq and Afghanistan.
The Parthians' reputation was well known to English-speaking scholars in the 16th and 17th centuries; for example, Samuel Butler makes a specific reference to their battle tactic in An Heroical Epistle of Hudibras to His Lady, 1678:
You wound, like Parthians, while you fly, And kill with a retreating eye.
The use of the actual term 'Parthian shot' comes rather later. As with 'parting shot', the literal use comes first. That is found in A Tour in India, the account of a Captain Mundy, who was Aide-de-Camp to Lord Combermere during a shooting trip to India in 1832. With all the bravery of those who club baby seals to death, Mundy describes his heroic encounter with a tiger:
Out rushed a little cub tiger of about three months, and charged me so courageously that my elephant took to her heels. I made a successful Parthian shot with my favourite Joe Manton [shotgun], and slew my determined little pursuer.
The metaphorical use of 'Parthian shot' comes soon afterwards, in The Times, April 1842:
They have probably enough dealt a Parthian shot to British interests, by setting the Nacional once more upon its legs.
Having two almost identical terms in the language which mean the same thing has led to the belief that one derives from the other. That may be the case, but there's no real 'smoking gun' evidence to link the two. 'Parthian shot' is unlikely to have derived from 'parting shot', as the military tactic it alludes to is so ancient. The fact that the earliest known examples of 'parting shot' pre-date those of 'Parthian shot' also tends to suggest that they were coined separately. We can't be sure, but is seems that the similarity between the two expressions is just co-incidence.