(Πίστιος), i. e. the god of faith and fidelity, occurs as a surname of Zeus, and, according to some, answers to the Latin Fidius or Medius Fidius. (Dionys. A. R. 2.49; Eur. Med. 170.)
[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