E. Rossoni notes that Di Gennaro derives from the proper name Gennaro, in turn originated in the Latin cognomen Januarius, probably applied to those born in the month of January. Its presence is concentrated in the centre and south of continental Italy, especially in the Naples, Caserta, Teramo, Foggia and Bari areas.

F. De Laurentiis agrees that the base of the surname is the name Gennaro, but broadens the etymological path: both the name and the common noun gennaio derive from the Latin Ianuariu(m), through the popular variant Ienariu(m). He explains that already among the Romans this name was originally imposed on children born in that month, dedicated to Ianus —"Janus"—, the god who opened the year.