The authors agree that the surname derives from the given name Tommaso, whose Latin form is Thomas, taken from the Greek Thomâs and in turn from the Aramaic t'oma', which meant "twin". E. De Stefani notes that the form Tomasin incorporates the diminutive suffix -ìn, with the original sense of "little Tommaso", and that over time it became a surname. He adds that Saint Thomas is the patron of bricklayers, architects, stonecutters and surveyors.

E. Rossoni documents the intermediate form Thomasinus, attested in various medieval deeds as a hypocoristic of the Latin name. Among the historical records he mentions a Thomasinus as abbot in Rome around 1225, Thomasinus de Grezano in the Greater Council of Verona in 1252, the surname Tomasini in Florence in 1432, and the student Scipio Tommassinus in Perugia in 1582.

As for geographical distribution, Rossoni places the form Tomasin in the Rovigo, Padua, Venice, Treviso, Udine, Gorizia and Trieste areas. De Stefani, for his part, documents a presence in the Carnia —a mountainous zone of northern Friuli, in the province of Udine—, specifically in Sostasio, a municipality of Prato Carnico, from the late 17th century.