The scholars consulted agree that the surname derives from Maso, a hypocoristic formed by aphaeresis from the given name Tommaso, already widespread together with its variants during the late Middle Ages. F. Miranda specifies that Tommaso traces back to the Hebrew-Aramaic term Tomà, meaning "twin," which was later adapted as Thomas in Greek and Thòmas in Latin, with the forms Thomasus and Tommasus appearing in the Byzantine period.

F. De Laurentiis adds an important qualification: in the dialects of the eastern Alpine area, maso also designates a rural property associated with livestock raising, so that in that region the surname may rest on a toponymic or occupational foundation entirely independent of the given name.

Miranda further notes that in southern Italy — particularly in Calabria, Sicily, and Apulia — the surname may alternatively derive from the Italianization of an arbëreshë surname (that is, a surname belonging to the Albanians of Italy) linked to an ancient clan known as the Mas Mat.

E. De Felice documents an extensive family of regional variants. The forms Masiello, Masillo, and Masullo are characteristic of the south and of the area of Naples; Masìn, Masiòl, Masón, and Masutti correspond to the Veneto; spellings with a double -ss- are typical of Tuscany; and Masina and Masotti are recorded in Emilia-Romagna. The surname Masi itself is most frequent in northeastern Italy and in Tuscany, while the base form Maso is found mainly in Campania and Apulia.