For starters, references:
The context it's described in, along with the keyword "Stout" to me, both suggest a thickness or stiffness that lends it to being a protective garment. Now, I believe most of us know that flexible leather, though it insulates well and protects well against thorns and other such things, doesn't do a whole lot when put up against a sharp blade. There's a reason we use broadheads on live deer. However, that's what the professor wrote, and I firmly believe that if he intended this "Jerkin" to be stiff, boiled leather, he would have described it differently. SO. For all intensive purposes, I believe the Jerkin placed on Merry is a stiff-er, but decidedly pliable, potentially (though not definitely) sleeveless, leather "vest" of sorts. I think the professor would have used the word Jacket if it had sleeves, since he uses the word elsewhere, but that's strictly conjecture, and isn't as firmly based as the rest of this.ROTK: The Muster of Rohan, @Rohan Armoury, Re: Merry wrote:'No mail have we to fit you,' said Éowyn, 'nor any time for the forging of such a hauberk; but here is also a stout jerkin of leather, a belt, and a knife. A sword you have.'
Moving on:
Frodo and Sam are talking about the orcish garments that they donned ("There were long hairy breeches of some unclean beast-fell, and a tunic of dirty leather.") upon leaving the Tower beyond Shelob's lair, so though they may be referring to a leather garment of orcish make, they wouldn't have any specific knowledge of Orcish tailoring/garments, so the word Jerkin must already be known to them from another context, and then be applied to the leather garments they are wearing as a way of describing them in their own words.ROTK: The Land of Shadow; Frodo/Sam: wrote:'Orc-mail doesn't keep these thorns out,' said Frodo. 'Not even a leather jerkin is any good.'
The trick is...they never passed into Rohan, so they hadn't seen any of the Rohirrim wearing garments similar to the one found in the armory and given to Merry. Being as little-traveled as they are, we can deduce that Frodo has encountered Jerkins:
1) In the shire, worn by hobbits. Unlikely.
2) Worn by the elves he has encountered on his rambles. Possible, but I'm not sold.
3) In use by Dwarves...perhaps not encountered in person, but perhaps described to him by Bilbo when telling stories of his travels (though there are no references to Jerkins in name in the Hobbit...but it at least leaves the possibility that Sam has heard the word in that context too, hence his understanding of the term).
4) Or, more directly, in their experiences with the Fellowship, meaning that there's a reasonable possibility that the Aragorn that they know, "...clad
only in rusty green and brown, as a Ranger of the wilderness" is being described as wearing brown leather, rather than brown wool/fabric.
That last one is all conjecture, of course, but it is plausible enough. In Rohan, it seems to be considered the lightest protective garment they have available, and Aragorn (here I go again!) is described primarily as a Traveler and Huntsman, so it isn't too far of a stretch, I would think.
I've long wrestled with "What's the point?" on the subject of the Jerkin. It adds another layer of insulation, and a plausible layer of light protection according to the Professor's design, whether we practically agree with its protective properties or not. "Story trumps rules", a wise man once said...
So is it worth wearing, and what would it look like?
Images are very sparse, since leather as a garment material was very rare through much of history, and punching in Jerkin doesn't start revealing workable results until well into the 16th century, and at that point, I feel the vast majority of period fashion well outside the scope of Middle-Earth-feeling styles...too fancy, over-tailored, etc., to be tromping about in the woods in. Their take on hunting and ours are decidedly different...more sitting on a horse and waiting for a dog or servant to flush game than the sleeping-in-a-ditch, muddy and spot-and-stalk experience we all cherish.
BUT, here's a start. A painting based on Christ's parable of The Blind leading the Blind from 1568 shows what I would assume are either meant to be earlier-period characters, or blind characters sporting old hand-me-downs due to their lack of social status, and the third gentleman from the left sports something that approaches what we tend to picture. Now, I can't vouch 100% for the accuracy of what they're wearing as per the period of the painting, as there are many of you on the forum that are far more the expert on the subject than I am, and it may well reflect true medieval style as accurately as our present-day Hollywood films show medieval culture well outside what was real, but at least it wasn't painted during this Millennium.
I wouldn't call this definitive, because I don't see the over-garment he's wearing as a protective piece at all (and why would it be, since he's blind? Not expecting to get in a tavern brawl, is he?) but it gives us a start as far as form/cut goes, and things like tassets or simply additional length could conceivably be worked into a middle-earth appropriate design without any real guilt, since it DID, at some point at least, exist in similar form, and it definitely existed as per the Professor's wishes, regardless of what today's top Fantasy artists have to say.