1667, first used by Milton (probably on analogy of cherub/cherubim), singular back-formation from O.E. seraphim (pl.), from L.L. seraphim, from Gk. seraphim, from Heb. seraphim (only in Isa. vi), pl. of *saraph (which does not occur in the Bible), probably lit. "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 Ar. sharafa "be lofty." Some scholars identify it with a word found in other passages interpreted as "fiery flying serpent."
I didn't know Milton was the first one to use this word in print. I love the imagery of a smoking or flaming angel- the smoke obscuring and trailing off, Lucifer falling from heaven like a meteor, shining and burning like a falling star.
Beautiful and terrible.