I don't know that I have ever eaten rhubarb, but I do know that movie makers use the word whenever they film crowd scenes.
Rather than have the crowd mill around and attempt to manufacture conversation, they are instructed to repeat "rhubarb" over and over.
A crowd of people repeating "rhubarb" sounds very much like a crowd engaged in numerous random conversations or commentary.
If you doubt me, the next time you watch a movie with a crowd scene focus your hearing on the sound of the crowd and ignore whatever dialogue may be going on among the actors. Before too long you will hear "rhubarb," clear as a bell.