Brewer's: Rupert of Debate
Edward Geoffrey, fourteenth Earl of Derby. It was when he was Mr. Stanley, and the opponent of the great O (i.e. O'Connell), that Lord Lytton so describes him. (1799-1869.)
The brilliant chief, irregularly great, Frank, baughty, bold- the Rupert of Debate.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894