A sugar plum has nothing to do with actual plums. They are called plums because they might be roughly the shape and size of one. A sugar plum consists of layers of sugar hardened around a center of seeds, nuts or spices. In fact, the sugar coated almonds known as Jordan almonds could be considered sugar plums. Traditionally, sugar plums were made in a very tedious and time consuming way. One batch might take days to layer all of that sugar. This meant that they were a candy only for the rich, assuring their legacy as a ‘real treat’.
Sugar plum came to mean many things in the English language. If your mouth is full of sugar plums it means that you speak sweet but perhaps false words. To stuff another’s mouth with sugar plums is to bribe them. Plum became British slang for one-hundred pounds. Someone who was rich could also be called a plum.
Of course, now with machines that do everything, candies that would have once been considered sugar plums are very affordable. Yet, the phrase still lives on meaning something delectable and desirable.