Green’s Dictionary of Slang

king of Spain’s trumpeter n.

1. a donkey.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1801		‘Modern Dict.’ in Sporting Mag. May XVIII .
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. a fool.

1834

‘Jack Downing’ Andrew Jackson 260: That are king of Spain’s trumpeter [...] who anounced tu you sich a hummer, about Mr Brown’s havin borrow’d a million from Squire Biddle’s bank when he didnt owe it a single dollar.