The tune of "Pop! Goes the Weasel" is a notorious earworm. It's a very simple tune, often played by children's toys, especially a jack-in-the-box. You might consider it to be like other nursery rhymes, going way back in time so that the original meaning of the words are obscure, and gaining a tune in more modern times. But that's backwards.
The song came first, in the 1850s. And it was a dance, too. And, like many other things from Christmas trees to white wedding gowns, it became popular because Queen Victoria liked it. The big dance craze of 1852 was a dance set to "Pop! Goes the Weasel." It was played as an instrumental except for the lyrics "pop goes the weasel," which came at particular point in the dance routine. And, as you might guess, when an instrumental tune becomes a hit, people will write lyrics for it so they can sing it. The early versions of those lyrics are quite different from what we sing today. Read about the origins and evolution of "Pop! Goes the Weasel" at Mental Floss.
(Image credit: Ferdinand Schuyler Mathews)
