The surname Maggi is widespread throughout the Italian peninsula and has multiple possible origins. According to the sources cited below, one hypothesis is that it derives from the medieval personal name Maggio, with Maggi functioning as a hypocoristic — that is, an affectionate or familiar form involving phonetic modification, such as shortening, apocope, or an endearing suffix — typical of family or dialectal usage.

R. Brondolo specifies that the underlying etymon may be the Latin Majus, meaning "greater," or simply Maggio as a name given to someone born in the month of May. A further hypothesis noted by Brondolo is that in certain religious communities the surname was assigned to foundlings born during May, following a pattern similar to the surnames Aprili, Giugni, and Settembrini, which likewise reflect the month of a child's birth or abandonment.

The consulted works concur in identifying a toponymic origin as another possibility: the surname may derive from places named Maggio, such as those found in the area of Como, or from Viamaggio and Viciomaggio in the province of Arezzo, in Tuscany. In Lombardy, some authors propose a dialectal etymology from màgg, an archaic Lombard term referring to nets or tools used for catching birds, which may have generated surnames associated with that traditional rural activity.

Finally, the sources suggest that the broad diffusion of Maggi across Italy may also reflect the deep cultural and symbolic resonance of May in Italian tradition: a month associated with the rebirth of nature and, in Catholic devotion, dedicated to the Virgin Mary — qualities that made it an appealing source for a positive or protective name. The surname is most concentrated in Lombardy, particularly in the areas of Milan, Pavia, and Brescia, and is also well attested in Tuscany and Emilia-Romagna.

Documented variants include Maggetti, Maggetto, Maggini, Maggino, Maggion, Maggione, Maggioni, Maggio, and Majo — the last of these using the letter j, which in medieval Italian represented the "long i" sound.