The authors agree that the surname Longo and its many variants derive from the Italian adjective lungo ("long, tall and rather thin"), originally used as a nickname for people of high stature and slender build. F. Miranda notes that the surname could also derive from variations, patronymic forms and hypocoristics of the medieval name Longus, besides the physical nickname.

E. De Felice documents the phonetic variants according to region: lóngo in the north and the south of Italy, luóngo in the Naples area, and slong in the Feltre area. The forms Luongo and Del Luongo are proper to the Naples area; Lungo and Del Lungo, to Tuscany; Slongo, to the Feltre area; Longhini, Longhìn and Longato are proper to Veneto; and Longoni and Longón, to Lombardy.

E. Rossoni documents the use of medieval forms such as Longus and Longettus as early as the 13th century, with examples in the Codice Diplomatico della Lombardia Medievale (1172), where the name Albertus Longus appears.

Some surnames derive from place names: De Felice and F. De Laurentiis agree that Longhini and Longhin, in Veneto, come from Longhi (in the Vicenza area), and that Longoni and Longón, in Lombardy, derive from Longone al Segrino, in the Como area.

V. Blunda supplies a dialect perspective: in Sicily, the surname comes from the Sicilian word llòngu ("tall, long, of great stature"), whose initial ll- gemination is characteristic of the Sicilian dialect as against the Italian lungo, and it could have originated as a nickname applied to a person of notably high stature.