The authors agree that the surname Greco derives from the medieval name or nickname Greco, based on the Latin term graecus, meaning “Greek” or “native of Greece.” Originally, it identified a person who came from Greece or, more broadly, from the eastern Mediterranean and Levantine regions. It could also refer to individuals connected with the Greek-speaking communities of southern Italy, heirs of the ancient traditions of Magna Graecia, or to followers of the Greco-Byzantine Christian rite.

G. P. Holmes notes that the surname’s earliest function was ethnic and geographical, indicating a person's origin. F. Miranda expands this interpretation by observing that the designation could also be applied to members of the grika and gricanica communities of southern Italy, whose language and culture preserve ancient Greek influences.

In addition to its ethnic meaning, the nickname greco acquired figurative uses in medieval and popular speech. According to the authors, it could describe a person regarded as clever, shrewd, elusive, or even deceitful, reflecting the old expression “to play the Greek,” meaning to pretend not to understand. P. Durante further notes that the term was sometimes used to refer to Albanians and other populations living across the Adriatic Sea who were culturally associated with the Greek world.

E. De Felice records numerous related variants, including Greca, Grecco, Greco, Grecu, Greci, Grieco, La Greca and Li Greci. The variant Grieco is especially common in Campania, Grego predominates in Veneto, and Del Greco is characteristic of Tuscany, while forms with articles such as Lo, Li and La are typical of southern Italy. The surname is widespread throughout Italy, with significant concentrations in Sicily, particularly in the provinces of Catania, Agrigento, Palermo, Messina and Trapani.

Overall, Greco is one of Italy’s most widespread surnames, originating primarily as an ethnic designation for people associated with the Greek world, but also acquiring cultural, religious and figurative meanings that contributed to its broad diffusion and numerous regional variants.