The authors agree that this surname derives from the Greek κάστανον ("chestnut"), a term that passed into medieval Latin and motivated the personal name Castanus, from which the surname was formed. E. Rossoni notes that the scholar Rohlfs links it directly to the Greek name Kàstanos, and documents its presence in the province of Matera and in the province of Messina, with the variant Castanò in the Catanzaro area.

G. Rohlfs specifies that the surname is of Greek-Byzantine origin and that the Greek etymon καστανός alluded originally to the chestnut colour of the hair. He documents it in the province of Reggio Calabria —in particular in Locri— and in the Soverato and Mesoraca area, in the province of Catanzaro, as well as noting its presence in the region of Campania. Rohlfs adds that the form Kastanòs survives as a surname in Greece.

V. Blunda, for his part, provides a complementary interpretation: the Greek term càstanon would also have functioned as a microtoponym, designating the tree or the terrain populated with chestnut trees, so that the surname could have been applied to the one who lived near a chestnut grove or came from that type of place. Blunda records it in Sicily and relates it to the variant Castanìa, of the same root.