In Tam o'Shanter, the hero (Tam) is on his way home from a convivial evening at the tavern with his "ancient, trusted, drouthy (thirsty) crony", Souter (Cobbler) Johnny, when he comes across a group of warlocks and witches having a party. They dance wildly, while the Devil plays the pipes (presumably the bagpipes). As the dancing gets more and more frantic, the witches "coost [their] duddies to the wark" (cast their clothes to the floor) and dance in their "sarks".
What's a sark? Well – there's the rub! We'll come back to this later ... for now, let's call it an underskirt.
Most of the witches are hideous old hags, but the one that catches Tam's eye is a "winsome wench and waulie (jolly)" whose sark is described as a "cutty-sark, o' Paisley harn" (a short underskirt of Paisley cloth). As he ogles this vision of loveliness, Tam gives thanks to the young witch's "reverend grannie", who bought her "Nannie" the sark for "twa pund Scots, ('twas a' her riches)" when she was a "lassie". The point is that the young Nannie has now grown up, and the sark is not long enough to maintain her decency when it's all she's got on (particularly when she's dancing and capering with gay abandon to the Devil's bagpipes).
This eventually gets the better of Tam, and he calls out "Weel done, Cutty–sark!". This unfortunately alerts the satanic company to his presence, and he is forced to flee for his life, on Maggie, his trusty grey mare (a.k.a. Meg). Knowing that witches can't cross running water, he heads for the bridge over the river; he makes it just in time. Nannie, who is at the head of the chasing throng, nearly catches up with him, but all she manages to do is to grab Maggie's tail – which comes away in her hand. And that's why the figurehead of the Cutty Sark (the ship) is clutching a horse's tail.
Let's now return to the burning question: "What is a cutty sark?"
Well ... I think we can dispense with "cutty" in fairly short order: it means "short". "Sark" on the other hand is a little harder to pin down. I've seen it translated variously as "underskirt", "skirt", "shirt", or "chemise". For example, this is from the blog Living with Scottish Clans and Castles, by one Alastair Cunningham: "But soon (25 Jan) it is Burns Night and I finish with my favourite witch: 'Nannie' famous for her 'cutty sark', short skirt, in Robert Burns' epic tale, Tam o' Shanter."
So Mr. Cunningham believes that a cutty sark is a short skirt; and so did I, last time I was asked the question. But my answer wasn't good enough; the answer required was "chemise or undergarment" (presumably either would have done).
I have researched this issue since. (Can you tell?) The source that I found most useful was this one. It gives the full text of Tam o'shanter, with a translation into English; it's where I got the translations I've used in this note from. It translates "sark" as "underskirt", but it includes an illustration where Nannie is wearing what to me looks like nothing so much as a nightdress. I later discovered that this is a representation (rendered back to front, somewhat bizarrely) of the 1856 painting by the Scottish artist John Faed – which you can see here. And other illustrators, possibly taking their lead from Faed, seem mostly to depict a similar sort of garment (try googling "cutty sark burns").
So I have to concede that a sark is probably not very much like a skirt as we know it today. The word "underskirt" conjures to my mind something like what my Mum would call a slip (which dictionary.com defines as "something easily slipped on or off" (definition no. 37!). I can't decide whether I find this definition rather coy or a bit suggestive; but anyway, I think it's a little harsh not to allow "a short skirt" as the answer to the question, when "underskirt" is the translation used by www.robertburns.org.uk, and a perfectly respectable Scottish blog calls it a "short skirt".
The next time this question came up in Macclesfield Quiz League (it generally comes up about once a year), the cutty sark was described as "a short petticoat". This strikes me as another perfectly reasonable term.
For what it's worth: if asked what a sark is, my advice is to answer "an undergarment". If asking the question, I think you should accept any of the options I've already mentioned: "undergarment", "underskirt", "skirt", "shirt", "chemise", "petticoat", and maybe even "slip" or "nightdress". (If the question is what's a cutty sark, the answer has to be qualified with "short".)
© Haydn Thompson 2017