How, exactly, did this phrase become a common turn of speech?
I know who the Parthians were, and that they were renouned for their skills in archery, so the fact of a Parthian shaft is no surprise. I know the Romans lost several significant battles against them, so that might explain the rankling, but that seems insufficient.
So, why does a Parthian shaft rankle so much? And how the heck did this expression become commonplace?
Is it common? Googling “rankle Parthian shaft” produces 27 entries, 3 of which are from Anne of Green Gables. A Parthian shot
“pointed or wounding remark made on departure, giving no time for reply” is a common phrase but rankling is not a word I’ve run across before.
And there you have your answer - use of the verbal “Parthian Shot” ( or “Parthian Glance” ) as a cheap debate technique or insult, is what rankles. It has nothing to do with actual Parthian arrows having been any more irritating than any others ( I’m sure all arrows are pretty annoying when it comes to that 
).
Okay then, so how did the debating technique become called after the Parthian arrows?
And maybe I think it’s common because I read Anne of Green Gables all the time. I just assumed that it was common.
In classical times, the Parthians were famous for their mounted archers who could riddle your best legion with arrows while riding away. Check out the sad fate of Crassusand his men (toward the bottom of the page).
"For the Parthians threw their darts as they fled, an art in which none but the Scythians excel them, and it is, indeed, a cunning practice, for while they thus fight to make their escape, they avoid the dishonour of a flight. "-from the link above.
I hope this makes reading Ann of Green Gables more rewarding for you.
Let me get this straight. You read this phrase in one book, and because you read that book all the time you assume the phrase is common? Wow. 
May the force be with you!
I think a Parthian shaft would rankle because they fire them at you as they seem to be running away. So, you think you have routed the Parthians, and you’re feeling pretty pleased with yourself, when suddenly you have an arrow stuck in you. That has to rankle!
I think the rankling part of it is that you can’t get your hands (or pilum), on the Parthian who just shot you to teach them a little Jovian respect. So in addition to the regular stinging from the arrow, you have all the frustration of unsated revenge. That rankles serious, you know?
The Romans were very caught up in the notion that the only proper way to deal with an enemy is to squash him with brute force. Trickery, deception, and guerilla tactics were thought of as cowardly. The Parthian shot frustrated Roman tacticians because it worked very well against them. And it just wasn’t right and proper that something so “unmanly” could be so effective.
I am quite aware of the Parthian tactics, and how the Romans dealt with them (ancient history major). But, frankly, they had a lot of other people that continually beat them too. I guess I’m just not very convinced there was anything super special about the Parthians.
And AndrewT, since I’ve read it several times over the period of several years, and since I don’t always recall exactly where I read something, I suppose it seems like I’ve read it in a lot of different places.
LaurAnge, it’s not that the Parthians continually beat the Romans, it’s that that one particular tactic really…well…got under their skin. The Romans didn’t like to be provoked, and the Parthian shot allowed them to provoke with great impunity. It was highly effective at luring them into ambushes, because the only appropriate response (to a Roman, anyway) was to pursue the bastards and run them through.
I think an appropriate comparison can be made between the Roman legions facing the Parthians and the British regulars being harassed by the American colonists at Lexington & Concord. In both cases, the smaller force knows it cannot win in a direct confrontation, and prefers to use guerrilla tactics. And in response, the larger, better equipped and better trained forces scream in frustration: “STAND STILL AND FIGHT LIKE A MAN YOU @*#% COWARDS!”
As to the expression, “rankle like a Parthian shaft”, I’ve never heard it before, but I knew exactly what it meant when I read this thread.
As in every game or war, when one side has clearly won, and the other side seems not to notice because they’re busy inflicting serious losses on the “winner”, the winner’s team is quite rankled.