Word-a-Day says this is a rallying cry for circus people.

Forty years or so ago, I read a book about a Romany or Gypsy family, travelng in a caravan wagon through the countryside. I don't remember the name of the book, but the author seemed to know a lot about Gypsies. There were a lot of Gypsy words and phases used in the book.

One of the phrases was "hey rube", which the author said was an
alarm call among them.