The authors agree that the surname derives from the personal names Giacomo, Giacobo, Giacopo and Giacobbe, the various forms by which Italian adapted, from the Middle Ages onwards, the Hebrew name Ya'aqob. In the Old Testament, that name corresponds to the patriarch Jacob, son of Isaac and Rebecca; in the New Testament, to the two apostles James the Greater and James the Less.

E. De Felice notes that the Hebrew name is formed by Yah —an abbreviation of Yahweh, "God"— and probably the root 'qb ("to protect"), with the combined meaning "God has protected". It was adapted in Greek as Jakob and in Latin as Jacob, giving rise to different forms according to the Latin or Greco-Byzantine tradition. He adds that Giacomo predominates in northern and central Italy, while in the south the patronymic Di is frequent; the variants in Ia- or Ja- are documented in the centre-south, and the abbreviated forms in Za- appear in the north.

F. De Laurentiis adds that the stress of Latin Jacobu(m) shifted to the preceding syllable under the influence of Greek Jákobos, and notes that the root 'qb admits diverse interpretations: "to protect" according to some authors, "to follow" or "to come after" according to others.