in reference to the winged, human-like celestial creatures that hovered above God's throne in Isaiah's dream, 1667, a word first used by Milton (probably on analogy of cherub/cherubim), a back-formed singular from Seraphim (attested from Old English). An earlier singular in English was seraphin (1570s).
This is from Late Latin seraphim, from Greek seraphim, from Hebrew seraphim (only in Isaiah vi), plural of *saraph (which does not occur in the Bible), probably literally "the burning one," from saraph "it burned."
Seraphs were traditionally regarded as burning or flaming angels, though the word seems to have some etymological sense of "flying," perhaps from confusion with the root of Arabic sharafa "be lofty." Some scholars identify it with a word found in other passages interpreted as "fiery flying serpent." The Late Latin word also was taken by early Christians as the name of a class of angels.