G. Rohlfs identifies this surname as of Greek-Byzantine origin, present in Catanzaro and Reggio Calabria, as well as in the province of Catanzaro (Guardavalle) and in the province of Reggio Calabria (Santo Stefano d'Aspromonte). He derives it from the Greek φυλακτόν, which means "amulet"; he also notes that in Greece the form Phylaktos survives as a surname. He warns that in Argentina it may lose the accent and appear written as Filasto.

P. Durante proposes, on the other hand, a different Greek origin: he relates it to philos ("love") or to khilós ("fodder"), without reference to the root φυλακτόν noted by Rohlfs. Both interpretations agree on the Greek origin of the surname, but differ as to the specific etymon.