The authors agree that the surname derives from casara or casèra, a term coming from the Latin casearia (from caseus, "cheese"), which designated the building or installation destined for the production of cheese in farms and stables. The trade form —that is, the name that designated the cheesemaker— varied according to the area: casaro, casàr, casèr and casè are the dialectal variants documented in northern Italy.

E. De Felice notes that the surname is also based on a series of place names derived from the same root, among them Casara in the Verona area, Casèra in the Belluno area, Casarile in the Milan area and Casaròla in the Verona and Parma areas. He indicates that the distribution goes from the Veneto, where the apocopated forms predominate, to Lombardy, where Casiroli is more frequent, and Emilia, where Casarini and Casaroli stand out. He adds that Casaroli, very common in Bologna, can also come from a nickname based on the Bolognese term casaròl, with the sense of "drunkard".

E. Rossoni specifies that Casari is concentrated in the north, especially in the Brescia and Modena areas, with presence also in Sardinia and Liguria; that Casarin is typically Venetian, above all in the Venice area; and that Casaro is located mainly in the Rovigo and Ferrara areas.