According to E. De Felice, Tossi, Tosso, Toso and their derivatives have a common origin in the dialectal noun tóso, which in the area of Veneto, Lombardy and Emilia-Romagna means "young" or "boy", and comes from the Latin tonsus ("shorn"), alluding to the custom of cutting the hair of the young.

These forms became surnames as early as the 11th century, with records such as Johannes Tosus in Farfa (1014) and Albizus Tosi in Florence (1321). Toso is more common in the north-east (Veneto and Friuli), Tosi in Lombardy and Tuscany, and Tossi appears as a more restricted variant, possibly influenced by the dialectal nasalization of Tosi or by phonetic deformation.

E. Rossoni indicates that these surnames present differentiated regional nuclei:

• Toso, Tosetto, Tosato, Tosatto are typical of the Italian north-east (Veneto, Trentino, Friuli).

• Tosi, Tosetti appear more in Lombardy and the centre-north of Italy.

• Tosso and Tossi may be linked to specific lines of the north, although Rossoni does not treat them as independent entries.

In the Dictionary of the Friulian Society of Buenos Aires, Toso is documented as an archaic byname that designated a child or young person. The surname Tós already appears in records of the year 1380 in Gemona del Friuli ("Lena figlia del Tos d'Artigna"), and is currently common in localities such as Buja, Udine, Monfalcone, Gorizia, Tarcento and Grado.

Some local variants preserve affectionate or diminutive suffixes:

• Toselli, Tosetti, Tosetto (north-east)
• Tosini, Tosino, Tosolini (north-centre)
• Tosatti, Tositti, Tosinghi (centre and Tuscany)