The authors agree that this surname derives from the medieval name Alegrus, of auspicious character. F. Miranda specifies its meaning as "may she be joyful and carefree" or "may she bring joy". E. Rossoni documents the form Alegrus in 1181, in the Diplomatic Code of Medieval Lombardy, and notes that the surname and its variants derive from that name directly or through diminutive and augmentative forms.

A. Bongioanni stresses that the original name has an auspicious character and that it has been used historically by Jewish communities, possibly as a translation of some Hebrew name of equivalent meaning. He also notes that, as a given name, it took root in Tuscany before extending towards Lombardy and Friuli.

As for the geographical distribution, Rossoni places the surname mainly in Sicily, while the variant Allegri is concentrated in Piedmont, Lombardy, Veneto, upper Tuscany and Emilia-Romagna. Miranda confirms the presence of the surname throughout Italy, with greater concentration in Sicily —where it extends across practically all its provinces— and in Piedmont, as well as specific areas in Lombardy, Lazio, Tuscany, Emilia-Romagna, Veneto and Liguria.