E. Rossoni notes that the surname Casi is characteristic of the Arezzo area, in Tuscany, while its variant Caso predominates in southern Italy (in the region of Campania, and in the Potenza, Foggia, Bari and Taranto areas). According to Rossoni, both forms would derive from a nickname linked to the Latin term caseum ("cheese"), which would have identified founders of lineages dedicated to cattle raising, shepherding or the making of cheeses.

V. Blunda proposes a different etymology for the context of Sicily: according to this author, the surname would come from the Greek casìas, meaning "tanner", and would have functioned as a professional nickname applied to the one who dedicated himself to leather work. Blunda relates it to the surname Casè, recorded in the same lexical field of the craft trades linked to leather.

The two proposals are independent of each other and refer to different regional traditions: the Latin etymology of Rossoni points to the centre and south of continental Italy, while the Greek one of Blunda is inscribed in the lexical substrate peculiar to Sicily.