E. De Felice classifies this surname as peculiar to northern Italy, with strong presence in the Milan area, and derives it from the place name Casirago, in the Como area.

E. Rossoni specifies that the surname is typically Lombard, concentrated in the Milan area and especially in the Brianza. He notes that Casirago, as the place name of origin, can be located both in the Como area and in that of Milan, and adds a historical fact: that locality was the starting point from which Barbarossa marched to destroy Milan. Rossoni also observes that the form Casirago as a surname is almost unique and probably the result of a transcription error, although he does not rule out that it may be a variant of the same surname.

Both authors agree that the origin of the surname is toponymic and refers to Casirago.