The sources consulted present two distinct etymological lines for this surname, which must be treated separately given the divergence between the majority position and a minority alternative.

E. De Felice traces the surname to the nickname and later personal name Magro, linked to bodily thinness — a physical characteristic that stands in contrast to surnames such as Grassi. De Felice further notes that the variants Magris and Magrìn predominate in Veneto, while Màgheri and Magherini are characteristic of Tuscany. F. De Laurentiis concurs on the essentials, confirming that the base is the nickname meaning "thin," applied in reference to the bearer's build, and that the surname is widespread throughout Italy.

E. Rossoni broadens this picture considerably. He specifies that the form Magri is typical of Lombardy and Emilia-Romagna; Magrin is concentrated in the area of Vicenza; Magrini is widely distributed across a central Italian belt comprising Tuscany, the Marche, Umbria, and Lazio; and the very rare Magrino appears in scattered localities around Vietri di Potenza, in the province of Potenza, and in the area of Udine. The variant Magro is very widespread in Veneto, particularly in the area of Padua, with additional presence in Sicily and Calabria. Rossoni traces all these forms back to the medieval personal name Magrus, derived from nicknames alluding to build, and documents it in a cartula libelli of the Codice Diplomatico della Lombardia Medievale of 1178, at Quinzano San Pietro — in the area of Varese — where a Petrus clericus qui dicitur Magrus is recorded. He also notes the surname's consolidation in Valtellina during the eighteenth century.

A divergent etymology is proposed by G. Rohlfs, who considers that Magrì may be a variant of Macrì, a surname of Greco-Byzantine origin derived from the Greek Makrìs, meaning "of tall stature." Rohlfs documents this form in Catanzaro and Reggio Calabria, as well as in localities of their respective provinces, and notes additional presence in Naples and Sicily. In his view, the form Magrì could result from the loss of the written accent on Macrì in migratory contexts.

A related but independently stated observation comes from the source Surnames in Italy, which considers the surname probably derived from a nickname based on the Greek adjective makròs, meaning "long, thin, tall," used to describe a person of great stature — a reading that partially overlaps with Rohlfs's Greco-Byzantine line without being identical to it.