In this case "the horns of the moon" (in waxing or waning quarters) represent an exaggeratedly high, elevated place, a lofty position where someone is figuratively placed. There are already published other versions less adjusted to the character of "motto" for a dictionary, such as putting someone on the horn the horns of the moon, raising someone on the horn the horns of the moon, lifting someone up on the horn the horns of the moon.
