SEMO SANCUS DIUS FIDIUS, an obscure Roman deity; a god Fisios or Fisovios Sancios was also worshipped in Umbria, and appears to be the same. He had a temple at Rome on the Quirinal, in which was an ancient statue of a woman, said to be Gaia Caecilia, or Tanaquil, wife of Tarquinius Priscus. His functions are very obscure. The four parts of his name seem to be connected respectively with seed (semen, cf. the Semunes in voked by the Fratres Arvales), purity or holiness (sancus and sanctus are from the same root), Jupiter (or simply brightness, or celestial nature; Dius, root Div), and faith (fides). Hence perhaps "the spirit of sowing (or seed), pure, Jovian (or bright, or celestial), faithful." Of his ritual we know only that oaths by him were taken in the open air, the formula being medius fidius, cf. mehercule, "by Hercules"; that there was an opening in the roof of his temple; and that certain discs of metal were kept in the temple. The first two facts perhaps suggest a sky-god, but
we know that certain deities undoubtedly not celestial (for ex ample, Terminus, the spirit of boundaries) had similar ritual. The discs might possibly be solar symbols. On the whole, the bal ance of the very scanty evidence is rather in favour of supposing him to be a god of the sky, perhaps connected, as Wissowa sup poses, with Jupiter, who had power to influence seed-corn (by sending rain in season?) and, being able to see what went on in the world, was a natural witness to solemn oaths. The ancients, wrongly supposing the last two members of his name to signify "son of Zeus," identified him with Hercules, a blunder which some moderns (as Preller) have fallen into.
See Wissowa in Roscher's Lexikon, s.v., and Religion und Kultus, 2nd ed., p. 129.