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A call for help. It originated among members of traveling circuses in the late 19th century. The carnival performer yelled out, "Hey, Rube!" as the unruly crowd advanced on him.
See also: rube
A rallying cry for assistance when trouble breaks out. The phrase began in the days of touring carnivals and circuses. A carnival or circus performer or stagehand who found himself in an argument or altercation with patrons or other outsiders yelled, “hey, Rube,” the signal for his colleagues to run and help him out. An item in the Chicago Tribune in 1882 explained that “a canvasman watching a tent is just like a man watching his home. He'll fight in a minute if the outsider cuts the canvas [to sneak in], and if a crowd comes to quarrel—he will yell, ‘Hey, Rube!' That's the circus rallying cry, and look out for war when you hear it.” “Rube” might have been the name of an actual person summoned for assistance, although another possibility is that “rube” referred, as it still does, to country bumpkins; that is, to members of rural carnival and circus audiences who were likely to start trouble.