Hey, Rube!

is a slang phrase most commonly used in the United States by circus and traveling carnival workers ("carnies"), with origins in the middle 19th century.

[1] Circuses were rowdy, loud, and often lewd affairs, where country people could gather, blow off steam, and voice political views.

is still the safety phrase used by many modern theatrical performers to alert security of a violent audience member, especially in outdoor or festival environments where entertainers are in close proximity with large numbers of intoxicated patrons.

The origin of the expression can possibly be traced to 1848 when a member of Dan Rice's troupe was attacked at a New Orleans dance house.

He'll fight in a minute if the outsider cuts the canvas, 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."