The consulted works concur in identifying two main lines of formation for this surname: derivation from a medieval personal name, and derivation from a descriptive byname or nickname.

A. Bongioanni reports the analysis of E. Rossoni, who describes this as a pan-Italian surname deriving in many cases from the medieval Latin personal name Morellus. This name is documented as early as the twelfth century: Rossoni cites an example dated 1171, preserved in the Annales Pisani, where a Morellus appears among the protagonists of a Pisan naval expedition toward Sardinia. In this respect, the surname became fixed as a hereditary form through the transformation into a surname of a medieval anthroponym, a process common in late-medieval Italy.

It is also recorded that the surname may be understood as a diminutive of Mauro, a view consistent with the broader etymological family identified by P. Lastrico and V. Blunda, who link the surname to the semantic field of moro / morello. This term was used historically to describe a person of dark complexion or dark eyes, or an animal with a black or very dark coat, particularly horses. Lastrico notes that this family of surnames derives in part from the old Latin name Maurus and in part from the medieval Italian adjective moro, which in the historical context of the Mediterranean could also carry the figurative sense of "Saracen." Blunda confirms the descriptive use in the dialect sphere, where morello designates specifically the black-coated horse, reinforcing the hypothesis of a physical or observational nickname as the origin of the surname.

Rossoni further notes that in some specific cases the surname may derive from place names containing this element, such as Ceppo Morelli in the province of Verbano-Cusio-Ossola, Dodici Morelli in the area of Ferrara, Poggio Morello in the area of Teramo, San Morello in the area of Cosenza, and Sasso Morelli in the area of Bologna, where it would have identified people native to those localities.

According to Lastrico, this surname belongs to a broad onomastic family rooted in the same etymological core of Maurus / moro, alongside parallel forms such as Moro, Mora, Moretti, Morelli, Morellato, Morellini, Moret, Moretto, Moroni, Morone, Mori, and Morisco. These are not direct variants but independent formations sharing the same semantic and cultural origin, widely attested across medieval Italy.