Φῆλιξ, an agnomen, having, like Magnus and Augustus, a personal rather than a general or family import. (Senec. De Clement. 14.) It was given to the dictator Sulla, and became a frequent addition to the imperial titles, being probably borrowed from the formula "felix faustum."
[W.B.D]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