The children were nestled all snug in their beds,
While visions of sugar-plums danced in their heads...
These famous lines you will surely recognize from the beginning of one of America's most beloved poems, Clement Clark Moore's "A Visit from St. Nicholas," a.k.a. "Twas the Night Before Christmas" (1823). I know the poem by heart. But I always snag on the part where the children nestle in their beds. What, exactly, is a sugar plum?
Please do not, dear reader, consult Wikipedia for wisdom on this matter, as was my first instinct. There you will find reflected only the contemporary prejudice linking sugar plums to actual fruit. It is understandable that well-meaning frutivores would assume that the traditional practice of preserving plums by boiling in sugar would result in something that would obviously correspond to the term. But this, from the perspective of history and tradition, is a confectionery travesty.
Please do not, dear reader, consult Wikipedia. There you will find reflected only the contemporary prejudice linking sugar plums to actual fruit.
The truth of the matter is that the sugar plum is not a plum at all, nor does it contain any plum-like substance. The sugar plums of Christmas fantasy are in fact sugar, and any resemblance to plums is entirely superficial.
If you don't know what a sugar plum is, you're in good company. The Oxford English Dictionary declares the term obsolete, and so it is. "Sugar plum" was well known to English-speakers from the 17th to the 19th century as another name for what was sometimes called dragee or more commonly comfit. I suspect that doesn't really clarify matters. All of these terms name a sweet made of sugar hardened around a central seed or kernel in successive layers using a process called "panning." The glossy sugar shells on candies like jelly beans or M&Ms are produced through a similar process: The candy pan is kept in motion over heat, while successive layers of sugar are poured on and allowed to harden. Jawbreakers are made this way, using a sugar crystal as the seed. Sugar plums or comfits were most often made with caraway or cardamom seeds at the center. Almonds were another classic base for sugar plum; the resulting candy would be like what we call Jordan almonds.
Confectionery historian Laura Mason calls comfit-making "one of the most difficult and tedious methods in craft confectionery, requiring specialized equipment, careful heat control, and experience." Depending on the size of the finished product, a batch could take several days to complete. Not just anybody could make these candies. Until the advent of machine innovations, comfits or sugar plums were a luxury good, most likely to be found in an aristocrat's pocket or between courses at a banquet.
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A little digging around in the OED gives some hints as to the elasticity of the word plum in the expression sugar plum. It names the fruit, of course, and the first sugar plums were likely named by association with the similar size and shape of nature's plums. But as sugar plum passed into general usage in the 1600s, it came to have its own associated meanings quite apart from fruit. If your mouth was full of sugar plums, it meant that you spoke sweet (but possibly deceitful) words. If you stuffed another's mouth with sugar plums, that meant a sop or bribe that would shut someone up. In the 18th century, plum was British slang for 100 pounds, or more generally, a big pile of money. And someone who was rich could also be called a plum. By the nineteenth century, plum has come to mean an especially desirable thing, a prize, a choice job or appointment. Trollope spells out the metaphor: "The chances are she won't have you—that's of course; plums like that don't fall into a man's mouth merely for shaking the tree." As for the more recent U.S. usage of plums to refer to testicles, I will leave you to your own armchair etymologies.