By long odds. By a great difference; as, “He is the best man by long odds.” A phrase used by betting men. In horse-racing, odds are offered in bets on favourite horses; so, in the Cambridge and Oxford races, long odds are laid on the boat which is expected to win.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Odds from Infoplease: