E. De Felice notes that this surname can reflect the medieval denomination "house of God" —in Latinised forms, casa Dei or, in northern Italy, ca Dei—, applied to hospices and religious institutions for orphans and foundlings. In that sense, it would represent one of the many names given to these children, like surnames such as D'Ignoti, Esposito, Innocenti or Proietti. De Felice adds that it can also be a place name derived from one of those hospices, such as Casadìo in the Bologna area, Cadèo in the Piacenza area, or Cadè in the Mantua and Reggio Emilia areas.

The Surnames in Italy source specifies that Casadei is concentrated especially in the Romagna —Cesena, Forlì, Ravenna, Rimini—, an area with a solid religious welfare network. Surnames such as Casadio, Casadidio, Cadei or Caddeo are considered variants of the same institutional model, regionally adapted.

E. Rossoni places these surnames in Emilia and Romagna and links them with nicknames derived from pious works, probably orphanages. Rossoni documents the first traces of its transformation into a surname in the Diplomatic Code of medieval Lombardy, in Milan, in the year 1188. De Felice, for his part, notes that the form Caddeo or Caddeu is peculiar to southern Sardinia and was probably imported from Genoa during the Middle Ages.