The authors agree that this surname derives from hypocoristic forms of the medieval name Thomasus. E. Rossoni notes that the form Tomaselli is diffused throughout Italy, while Tommaselli is concentrated in the Naples and Benevento area. Among the less frequent variants, Rossoni mentions Tomasello, of probable Sicilian origin, and Tommasetti, very rare, present in central Italy —in the regions of Marche, Abruzzo, Molise and Lazio— as well as in the Naples area. The forms Tommasello and Tommasetto are practically non-existent.

F. De Laurentiis links it specifically as a derivative of Tommasi, which complements the chain of formation: from Thomasus first Tommaso would have been generated, then Tommasi as a surname, and from this the hypocoristic forms in -elli, -ello, -etto and -etti.