Green’s Dictionary of Slang

a call for help.

1879		Cincinnati Enquirer 7 Sept. 10/7: ‘Hey! Rube!’--The circus-man’s shout, which has been heard from Maine to Oregon and from Hudson’s Bay to Brazil. When the countrymen get too fresh and too full of fight, they generally get it. The first performer attacked sends forth the thrilling war cry, and every man and boy connected with the show arm themselves with some weapon, and sally to the aid of their brother.
1891		H.C. Bunner Short Sixes 100: If the boys was here, and I hollered ‘Hey Rube!’.
1906		S. Ford Shorty McCabe 63: We knew that the one who sent it meant to ring up a hurry call on us for help [...] ‘That’s a Hey Rube for me,’ says I.
1921		Conklin & Root Ways of The Circus 241: Red hot and eager for the fray; / The boys all thought, but didn’t say—‘Hey, Rube!’.
1924		F. Isman Weber and Fields 100: Clems, as such shindigs were known in circus argot, and their battle cry, ‘Hey Rube!’.
1931		R.E. Howard ‘Circus Fists’ in Fight Stories Dec. 🌐 I seen Joe take it on the run, ducking out under the wall of the tent, and yelling, ‘Hey, Rube!’.
1950		Goldin et al. DAUL 95/1: Hey, rube! (Carnival) ‘Help!’ A common call for assistance in a brawl on the carnival grounds.
1971		(con. 1920s) J. Brown Monkey Off My Back (1972) 18: If such hostility seemed afoot, the first carney to recognize it would yell, ‘Hey, Rube!’.
1986		D. Barker (con. 1925) Life in Jazz 117: ‘[O]n circuses, carnivals, medicine shows, all concession owners are called Rube, and when anybody connected with the show gets in trouble with [...] people from the town, he hollers, “Rube!” and the show people rush to his defense and rescue’.
1998		J. Lansdale Rumble Tumble 131: ‘Hey Rube’ was yelled, and Herman was forced to kick some butt.
2012		W. Keyser ‘Carny Lingo’ in http://goodmagic.com 🌐 Hey Rube! — In the 'old days,' a call for help when a carny encountered more trouble with outsiders than he can handle alone. These days [...] it's more likely to be ‘It’s a clem!’ or [...] simply ‘"fight!’.