From Middle English abrod, equivalent to a- + brood.
Meanings
(obsolete) Upon a brood; on a hatch.
The word in the original (as St. Hierom tells us from the Hebrew traditions) implies, that the Spirit of God sat abrood upon the whole rude mass, as birds upon their eggs, […] 1821, George D'Oyly, Hendrik Slatius, Henry Wharton, The life of William Sancroft, Archbishop of Canterbury: