The authors agree that the surname, together with its variants Castorino and Castorini, derives from a dialectal nickname based on the archaic term casturinu, a thick woollen cloth used to protect oneself from the cold during nocturnal or winter shepherding, which suggests that the first bearers of the surname would have been shepherds.

E. Rossoni places the surname mainly in the Catania area —with presence in localities such as Acireale, Aci Castello, Giarre, Aci Catena, Gravina di Catania, Riposto, Misterbianco, Zafferana Etnea, Aci Sant'Antonio and San Giovanni la Punta— and in the Messina area, including Taormina. He also notes that the variant Castorini appears in the Naples area and in the Brindisi area, and that Castorino is recorded in the Messina area.

F. Miranda highlights the typically Sicilian character of the surname and broadens the geographical distribution within Sicily, adding presence in the Syracuse area —with localities such as Lentini and Floridia—, in the Palermo area, in the Enna area, and in the Ragusa and Agrigento areas. He also indicates that the surname is found outside Sicily, in regions such as Lombardy, Piedmont, Lazio and Tuscany.