The word 'pleb' is Latin and means a member of the plebeian, or common class - something like working-class but much broader. In Ancient Rome plebs were free citizens, who could own land and often became rich but were regarded as lower class compared to the patricians, who were the elite group and held much of the political power. Over time the word 'pleb' began to be used as an insult to imply a low or uncouth person.

The word has been in the news lately because Tory Chief Whip Andrew Mitchell is alleged to have shouted the word at a police officer who wouldn't let him ride his bike where he wanted. He is also supposed to have said that the police needed to learn their place. Some members of the police force have said Mitchell should resign, both because it's important for MPs to show respect for the police and because, by denying he used these exact words, he is basicaly accusing the officer of lying.