E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Horn of Fidelity.
Morgan la Faye sent a horn to King Arthur, which had the following virtue:No lady could drink out of it who was not to her husband true; all others who attempted to drink were sure to spill what it contained. This horn was carried to King Marke, and his queene with a hundred ladies more tried the experiment, but only four managed to drinke cleane. Ariostos enchanted cup possessed a similar spell. (See CHASTITY.)