E. Rossoni proposes several hypotheses on the origin of this surname. The first derives it from the medieval name Napo —which recalls the celebrated Napo dei Torriani, lord of Milan, defeated by Ottone Visconti in 1277 in the battle of Desio—. The second hypothesis links it to the Germanic term knapp, with the meaning "cup", "glass" or "goblet", possibly in allusion to the activity of production or trade of those objects. Rossoni further adds that the surname may refer to an ornament with tassels or function as a nickname for a person with a prominent nose. He considers it typical of the region of Campania, especially at Naples, Salerno and their respective areas.

V. Blunda offers a different reading and places the surname in Sicily, where he considers it of dialect origin. He derives it from the Sicilian word nnàppa, with the meaning "naive, guileless and easily deceived person", that is, someone who lets himself be tricked without suspicion. According to Blunda, the surname would have arisen as a nickname of a behavioural character, applied to a person of excessive kindness or of little cunning in dealings.