The Latin metrical form of the hymn was composed as early as the 12th century.
Each antiphon begins as follows:

O Sapentia (Wisdom)
O Adonai (Hebrew word for God)
O Radix Jesse (stem or root of Jesse)
O Clavis David (key of David)
O Oriens (dayspring)
O Rex genitium (King of the Gentiles)
O Emmanuel

Put together, the first letter of the second word of each antiphon spells SARCORE. If read backwards, the letters form a two-word acrostic, “Ero cras,” meaning “I will be present tomorrow.”