The authors agree that the surname derives from the name Giulio, which continues, in a semi-learned tradition, the ancient Latin nomen Iulius (with regional variants such as Zulio and Iulio). E. De Felice specifies that this name comes from *Iovilios, in turn derived from Iovis ("Jupiter"), with the original meaning "sacred, dedicated to Jupiter". He further notes that in Virgil's Aeneid Iulus appears as the name of the son of Aeneas, the mythical ancestor of the gens Iulia of Julius Caesar.

E. Rossoni indicates that Giuli is proper to central Italy, with a presence in the provinces of Rome and Rieti, and specific areas in Umbria and the Marche. Other related forms show varied distributions: Giulietti is characteristic of the Marche, Umbria and the province of Rome; Giulioni is concentrated in the Marche, with a lesser presence in the Perugia area; Giulini, very rare, appears in areas of Lombardy, Pesaro and Rome. Rossoni also recalls the Milanese historian Giorgio Giulini (1714–1780), author of a history of Milan.

De Felice, for his part, notes the existence of patronymic forms in -ich proper to the Venezia Giulia area, and records Iuli as a sporadic form of southern Italy.