an ancient form of filius, occurs in the connection of Dius Fidius, or Medius fidius, that is, me Dius (Διός) filius, or the son of Zeus, that is, Hercules. Hence the expression medius fidius is equivalent to me Hercules, scil. juvet. (Cic. Fam. 5.21; Plin. Ep. 4.3.) Sometimes Fidius is used alone in the sense of the son of Zeus, or Hercules. (Ov. Fast. 6.213; comp. Varro, de L. L. 5.66; Plaut. Asin. 1.1. 8; Varro, apud Non. 8.93.) Some of the ancients connected fidius with fides. (Festus s. v. medius.)
[L.S]A Dictionary of Greek and Roman biography and mythology
Smith, William
A Dictionary of Greek and Roman biography and mythology. William Smith, LLD, ed. 1890