The scholars consulted agree that the surname derives from the medieval given name Lello (feminine Lella), a hypocoristic form of names ending in -ello / -ella, such as Raffaello, Gabriello, Donatello, Brunello, and Leonello, among others. This origin is also confirmed by a third source, which similarly traces the surname to Lello as a diminutive of names ending in -ello, citing Angiolello, Brunello, and Raffaello as examples.

E. De Felice notes that the surname has a greater presence in Emilia-Romagna and Tuscany, while feminine forms and patronymics formed with De are more characteristic of southern Italy.

E. Rossoni adds a medieval documentary reference: in the second half of the thirteenth century, a notary designated as De Lellis is recorded in the area of Avellino, which places the use of these forms in southern Italy at an early date. Rossoni also indicates that the variant Lelli is widely spread across central Italy, while Lellis is extremely infrequent and Lello is very rare, with a probable origin in the South.