The authors differ on the origin of this surname. F. Miranda proposes several possible etymologies: it could derive from contractions and dialect variants of the name Jacopo or Giacomo; from the medieval word giacco, which designated a kind of mail coat used as body protection by some soldiers; from the Italianization of the French name Jacquin; or from the Hebrew name Jakim. Miranda notes that the surname is distributed across numerous regions of Italy, among them Piedmont, Sicily, Lombardy, Tuscany, Liguria, Lazio, Emilia-Romagna, the Aosta Valley, Apulia, the Marche, Sardinia and Friuli-Venezia Giulia.

V. Blunda, by contrast, proposes an independent dialect origin for the Sicilian variant: he derives it from the Sicilian word giacùni, meaning "large boulder, big stone". As a surname, it would have arisen from a nickname applied to a person of large and compact build, or else from a microtoponym referring to a spot marked by some conspicuous crag. The augmentative suffix -ùni reinforces the image of considerable bulk.