E. Rossoni puts forward two hypotheses on the origin of the surname. The first links it to a nickname or auspicious name derived from alleanza ("alliance"), alluding to someone who brought an important support by way of dowry. The second relates it to the medieval feminine maternal name Allegrantia (also Allegranza), through deformation and aphaeresis; this form is documented in a Carta vendicionis of 1159 in Vimercate, in the Milan area.

G. P. Holmes proposes, for his part, that the surname derives from the Sicilian lianza, with the sense of "of the covenant", and suggests a possible Jewish religious connotation: "he who belongs to the covenant between God and the Jewish people". Holmes also adds an alternative derivation from the Old Italian leanza, meaning "loyalty".

Geographically, Rossoni identifies as the main area of concentration of the surname the region of Sicily, in particular the Catania area (Adrano, Catania, Bronte, Paternò and Biancavilla). A presence is further recorded in the Salento area —the southernmost zone of the region of Puglia, in southern Italy— (Scorrano and Maglie, in the Lecce area), as well as in Casalnuovo di Napoli. The variant Leanzi is practically unique and is probably attributable to a transcription error.