A cowbell is a small metal bell. The first job it had was a farm job: you hang it from a cow’s neck, and when the cow walks the bell rings, so you can hear where she is in a field, in mist, or in the trees. That is still what the word names. Musicians later picked the same object up and used it as an instrument — you hit the bell with a stick and it gives a dry, clanking ring. In bands and in school percussion it is still called a cowbell, even when no cow is anywhere near. People also say it with a smile (“more cowbell”) when they want a loud, slightly silly extra sound. It is an ordinary, slightly folksy word, not a technical one, except in music class.