The authors agree that the surname derives from a physical, descriptive byname linked to the Italian term magro ("thin, lean, spare"), from the Latin macer. E. Rossoni specifies that the starting point is the medieval name Magrus, attested as a late Latin anthroponym, applied to an ancestor characterized by his thinness. According to Rossoni, the plural diminutive suffix -ini, with an affective value, would originally have designated the descendants or the younger members of the clan of the one called Magro. F. De Laurentiis sums up the etymology simply as derived from "thin".

As for the geographical distribution, F. Miranda notes that the surname is found in more than four hundred comuni, mainly in regions of central and northern Italy: Tuscany, Lazio, the Marche, Emilia-Romagna, Umbria, Lombardy, Veneto, Friuli-Venezia Giulia and Liguria. In Sicily it appears only in a few comuni of the Catania, Caltanissetta, Trapani and Enna areas. Rossoni agrees on the central distribution and indicates that the variant Magri is typical of Lombardy and Emilia-Romagna, Magrin corresponds to the Vicenza area, Magrino is very rare —with a scattered presence in Vietri di Potenza and in the Udine area—, and Magro is very widespread in Veneto, especially in the Padua area, with clusters also in Sicily and Calabria.