E. Rossoni and F. De Laurentiis agree that this surname derives from the medieval name Gennaro, which in turn comes from the Latin cognomen Ianuarius. De Laurentiis adds that the Latin form passed into popular use through the variant Ienarium, and that already among the Romans this name was originally imposed on children born in January, the month dedicated to Ianus —the god who opened the year—.

As for the geographical distribution, Rossoni notes that the surname is present throughout the centre-north of Italy and in the Salento area —that is, the southernmost zone of the Apulia region, in southern Italy—, with a notable presence in the Pesaro area and in Lazio. De Laurentiis, for his part, places it mainly in the centre of Italy and in the south. The form Gennarini would seem to be typical of the Rome area, while Gennarino is extremely rare. The form Gennaro, on the other hand, presents zones of diffusion in Sicily, in the Salento area, in the Naples area, and also in Veneto and in Friuli.