The authors agree that the surname derives from a nickname linked to the dialect word ratt or ratto, related to the rodent, although they differ in the interpretive nuances and in the scope of its origin.

E. De Felice notes that the nickname alludes to the rat, in particular the "sewer rat," with dialect forms such as the Ligurian ratu —with the augmentative ratassu— and the Lombard rat, applied in reference to the agility and speed of the animal. He documents the name or nickname in Genoa in 1159 under the form Ratus, and places the greatest frequency of the surname in Liguria and Lombardy.

E. Rossoni broadens the possibility of a toponymic origin, mentioning places such as Pian dei Ratti, in the Genoa area, and Borgoratto and Torre de' Ratti, in the Alessandria and Pavia area. He identifies a main nucleus in Lombardy, concentrated in the Milan, Bergamo, Como and Lecco areas, and another in the La Spezia and Massa Carrara area.

R. Brondolo adds that in lower Piedmont the connection with ratto or topo ("mouse") does not necessarily have a pejorative value —it can be interpreted as "fast as a mouse"— and further links the term with a toponymic element that designates an "inhabitant in a steep and wooded place," a use documented in western Lombardy and in the Swiss region of Ticino.