The authors agree that this surname is of toponymic origin, derived from Merate, a locality currently in the province of Lecco, in the region of Lombardy. E. Rossoni notes that the place name goes back to the ancient designation Melatum, with the meaning "place of apple cultivation", a crop widespread in that area from remote times. E. De Felice classifies it as a surname proper to upper Lombardy, formed directly from that place name.

Rossoni supplies the oldest documentary testimony of its fixing as a surname: a reference in the Codice Diplomatico della Lombardia medievale, in a deed of investiture of 1188 in Milan. The surname is typical of the north of the province of Milan and is historically associated both with that city and with the Brianza area.

The Merati family is held to have been present in Milan and the Brianza from early times, with documents of the 13th century mentioning it in functions of representation, council and administration in Milan. In the 14th century, the family built a chapel in the church of the Santo Sepolcro in that city.