A rube is someone from the country, or someone who thinks in a simple way and has not seen much of the world — and the speaker looks down on them. It is informal American English, a little old-fashioned now, and always unkind. City people used it for farmers and small-town visitors who might be cheated at a fair or a circus. It comes from Rube, a nickname for the man's name Reuben, which used to sound like a country name. There is also an old circus cry, 'Hey Rube!', shouted when circus people needed help in a fight with the town; the 'rubes' were the locals. Do not call a person this to their face; it is an insult.