The consulted works concur in tracing this surname to a nickname applied to ancestors who were notably hairy or had abundant hair. E. De Felice specifies that it derives from the adjective peloso, meaning "one who has thick and long hair or beard," and documents the form Pelosus or Pilosus in Genoa as early as 1159. He also identifies regional variants: peluso in southern Italy and pilusu in Sicily. E. Rossoni concurs with the core derivation from the same nickname. It is likewise recorded as deriving from peloso in the sense of someone with long hair or beard.

Geographically, De Felice notes that Pelosi is sporadic across Italy, while the related forms Peloso and Peluso are frequent in southern Italy and particularly in the area of Naples. The variants Piluso is concentrated in Sicily and Calabria.