The authors agree that the surname was formed from the proper name Tommaso, whose ultimate etymon is the Aramaic t'ōmā', meaning "twin". This term was adapted into Greek as Thomas and then into ecclesiastical Latin as Thomas or Thomasus, a form from which the medieval variants Tommasus and, finally, Tommaso or Tomaso derive.

E. De Felice notes that the diffusion of the base name was due to the prestige of the cult of Saint Thomas the apostle from the first centuries of Christianity and, from the 13th century onwards, of Thomas Aquinas and Saint Thomas of Celano. He further indicates that the surname is of pan-Italian distribution, with a greater presence in the north-east of Italy —in the regions of Veneto, Friuli-Venezia Giulia and Trentino-Alto Adige—, in Emilia-Romagna, Tuscany and in the Salento area —that is, the southernmost zone of the Apulia region, in southern Italy. He also refers to the related surnames Masi, Tomei and Fumei.

F. De Laurentiis adds that the Aramaic nickname would have served to distinguish the apostle from another person of the same name, which would explain its origin as a byname before consolidating as a proper name.