Brewer's: Catholic King
(The) or His Catholic Majesty. A title given by the Pope to Ferdinand, King of Aragon (1452, 1474-1516), for expelling the Moors from Spain. This was about as unwise as the revocation of the Edict of Nantes by Louis XIV.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894