halo n.

1560s, "ring of light around the sun or moon," from Latin halo (nominative halos), from Greek halos "disk of the sun or moon; ring of light around the sun or moon" (also "disk of a shield"); "threshing floor; garden," of unknown origin. The sense "threshing floor" (on which oxen trod out a circular path) probably is the original in Greek. The development to "disk" and then to "halo" would be via roundness. Sense of "light around the head of a holy person or deity" first recorded 1640s. As a verb from 1791 (implied in Haloed).