rhubarb

Chris Glass did some ‘rhubarb’ wiki’ing and found these uncited tidbits about the stalky red vegetable:

It is or was common for a crowd of extras in acting to shout the word “rhubarb” repeatedly and in an unsynchronised manner, to cause the effect of general hubbub. As a result, the word “rhubarb” sometimes is used to mean “length of superfluous text in speaking or writing”, or a general term to refer to irrelevant chatter by chorus or extra actors. The American equivalent is walla. Stage actors in the United States also use word “rhubarb” repeated asynchronously in a low or murmured tone to provide background voice ambience in crowd or party scenes. A variation of this is the repetition of the phrase “peas and carrots”
– wiki : rhubarb