You might be familiar with a nursery rhyme about three men in a tub. Or the BBC’s version about socks (if my socks had that many holes, I would just get rid of them).
Rub-a-dub was originally a word for the sound made by beating a drum or for a drumming sound.
It is often used to describe a rhythmic sound or motion, such as rubbing or scrubbing to make something clean.
Rub-a-dub is also rhyming slang for pub.