E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Horns of the Altar (To the).
Usque ad aras amicus. Your friend even to the horns of the altari.e. through thick and thin. In swearing, the ancient Romans held the horns of the altar, and one who did so in testimony of friendship could not break his oath without calling on himself the vengeance of the angry gods.