Green’s Dictionary of Slang

a fight, orig. between circus or carnival people and local townspeople; also attrib.

1899–1900		C.L. Cullen Tales of the Ex-Tanks 280: the circus had got mixed up in a ‘Hey, Rube!’ battle a couple of towns up the line.
1922		T. Thursday ‘Ten Dollars – No Sense’ in Top-Notch 15 Dec. 🌐 The picnic reminded me of a ‘Hey, Rube!’ on the circus lots. All that I needed to make me feel comfortable was a six-foot stake.
1939		in A. Banks First-Person America (1980) 204: A Hey Rube was a fight between the circus folks and the town yokels.
1946		J.E. Dadswell Hey, Sucker 102: hey rube ... fight between show people and town people.
1946		W.L. Gresham Nightmare Alley (1947) 72: You haven’t any more sense than to start a Heyrube.
1956		H. Gold Man Who Was Not With It (1965) 6: They were a party; we found ourselves with an old-fashioned hey-rube and obliged to move the show on that night.
1962		C. Clausen I Love You Honey, But the Season’s Over 180: Minnie ain’t so young any more and we have a ‘Hey, Rube’ every time I talk to a broad.
1974		R. Blount Jr About Three Bricks Shy of a Load 8: In Pittsburgh they call a brawl a ‘hey-rube’.