E. Rossoni proposes that this surname would derive, directly or through diminutive forms, from the name Fenzo, a contracted form of Fiorenzo or of Fidenzio. As an illustrative reference he mentions the parish of San Fenzo (Fidenzio) in Baone, in the Padua area. Rossoni documents the transformation into a surname in Venice towards the second half of the 18th century, with Modesto Fenzo, a recognised typographer and publisher. He notes that Fenzi presents specific zones of concentration in the Genoa and Livorno areas, and between the Verona and Rovigo areas, while Fenzo is typically Venetian. The variant Fenzini is practically unique.

F. De Laurentiis proposes, on the other hand, a different etymology: the base would be the term fens, which designates an obstacle formed by a ditch followed by a hedge, which would probably indicate a zone of frontier or geographical boundary as the origin of the surname.