According to my father, "rhubarb" is what the extras say in crowd scenes in TV and film. They mumble, "rhubarb, rhubarb, rhubarb" and it gives the impression of muted, indistinct background conversations.

By the same token, the choral singer who forgets the lyrics during a performance can sing "watermelon, watermelon" until she gets to a place where she recalls the words. The philosophy is that, as long as you get a convincing vowel in there, people will believe anything you sing.