Originally a rube was a farmer character in a play. Later the term came to refer to any scene or show with a rural setting or atmosphere. The term was frequently employed, for example by George M. Cohan, who used the term in his song lyrics as well as in shows featuring a Broadway type dropped into a rural environment, as in his hit musical Forty-Five Minutes from Broadway (1906), among others.
The Historical Dictionary of the American Theater. James Fisher.