A derivative form from summus, the highest, an ancient Roman or Etruscan divinity, who was equal or even of higher rank than Jupiter; in fact, it would seem that as Jupiter was the god of heaven in the bright day, so Summanus was the god of the nocturnal heaven, and lightnings plying in the night were regarded as the work of Summanus (Augustin. De Civ. Dei, iv. 23; Plin. H. N. ii. 53; Paul Diac. s. v. Dium, p. 75; Fest. s. v. provorsum, p. 229, ed. Müller.) Varro (De Ling. Lat. v. 74) describes the god as of Sabine origin; but the ancients themselves on this as on many other points connected with their earliest religion, were in great uncertainty both in regard to the nature and the origin of Summanus ; and some connecting the name with sub and manes regarded him as a deity of the lower world, an opinion which is totally at variance with the attributes given him by most writers, and there is ample reason for regarding him as the Jupiter of night. He had a temple at Rome near the Circus Maximus (Plin. H. N. xxix. 14; Liv. xxxii. 29 ; Ov. Fast. vi. 731). There was a representation of Summanus in the pediment of the Capitoline temple (Cic. de Div. i. 10.)From Smith's Dictionary of Greek and Roman Biography and Mythology
From The Malleus Maleficarum of Heinrich Kramer and James Sprenger
Such deities were the Thracian Bendis, whose manifestation was heralded by the howling of her fierce black hounds, and Hecate the terrible “Queen of the realm of ghosts,” as Euripides calls her, and the vampire Mormo and the dark Summanus who at midnight hurled loud thunderbolts and launched the deadly javelin through the starless sky. Pliny tells us that the worship of this mysterious deity lasted long, and dogs with their puppies were sacrificed to him with atrocious cruelty, but S. Augustine says that in his day “one could scarce find one within a while, that had heard, nay more, that had read so much as the name of Summanus” (De Civitate Dei, iv, 23). Nevertheless there is only too much reason to believe that this devil-god had his votaries, although his liturgy was driven underground and his supplicants were obliged to assemble in remote and secret places.