The authors agree that this surname derives from the personal name Gennaro, which continues the Latin cognomen Ianuarius, related to the month of January. F. Miranda specifies that Ianuarius took its name from Janus, the two-faced Roman god who presided over doors and passages, so that the meaning is interpreted as "dedicated to Janus" or "born in January". Miranda also adds that, according to the Roman calendar —in force until the mid-2nd century AD—, January was the eleventh month of the year.

The authors agree that the diffusion of the name is linked to the cult of San Gennaro, bishop of Benevento, martyred in the year 305 in Pozzuoli and patron of Naples, which explains its massive presence in southern Italy and its scarce frequency in the rest of the country. A. Bongioanni and E. De Felice note that in Tuscany the form Gennai appears, and that in the south the patronymic variants with De or Di are frequent.

Bongioanni also identifies regional forms derived from the same name: Genero and Naretto in Piedmont; Zennari and Zonaro in Veneto; Zanier in Friuli; and Zenaroller and Zendrini in Trentino. Miranda, for his part, records as direct variants Di Gennaro, De Gennaro, Gennari, Gennarini and Gennarino, and documents a presence of the surname throughout Sicily, with a concentration in the Palermo, Catania, Syracuse, Messina, Trapani and Enna areas.