The authors differ on the origin of the surname. E. Rossoni notes that the surname shows multiple independent clusters of origin (ceppi), arisen in different regions without direct relation between them. The main core is Lombard-Emilian, with specific areas in Tuscany, Liguria and eastern Sicily. Rossoni proposes that it could derive from the medieval name Lossus, although he also considers possible a derivation from aphaeretic forms of medieval Latin names such as Colossus.

Rossoni also documents related formal variants. Loss is typical of Trentino, especially of Canal San Bovo and Fiera di Primiero. Lossi shows specific areas in lower Brescia and in the Lucca area. Loso, almost unique, would appear to be from Campania. Losso has a presence between the provinces of Bolzano and Belluno, and in the Cosenza area.

For his part, V. Blunda offers an alternative hypothesis of regional scope: he considers that in Sicily the surname has a dialect origin and comes from the word alòsa, meaning "almond". The fruit was a central ingredient of Sicilian confectionery of Arab-Norman tradition, and the surname could have arisen as a nickname applied to the gatherer or seller of almonds, or else from a microtoponym linked to a notable almond tree in the landscape.