personification of the west wind in Roman mythology, from Latin Favonius, which de Vaan suggests is cognate with the god-name Faunus (see faun), from a prehistoric noun meaning "who favors" (see favor (n.)):
This also yields a good semantic motivation: the wind that stimulates vegetation can be called favourable. Favonius was regarded by the Romans as the herald of spring and the start of new vegetation (e.g. Cato Agr. 50.1, Cicero Ver. 5.27, Lucretius 1.11, Vitruvius 2.9.1).
The Latin word is the source (via Old High German phonno, 10c., via Vulgar Latin contraction *faonius) of German Föhn "warm, dry wind blowing down Alpine valleys." Related: Favonian.