The authors agree that this surname derives mainly from the Italian term re ("king"), used as a nickname. E. De Felice details that this nickname could refer to the newborn celebrated as "king" of the family feast, to the one chosen "king" in gatherings or groups, to the winner of competitions —especially of archery or crossbow shooting in the medieval period— or to one who excelled in an art or trade. E. Rossoni adds that the nickname could also allude to those who served in the army or among the servants of a king, or to some characteristic of the bearing or character of the founder of the line.

De Felice further notes that, in northern Italy, the surname may have a toponymic origin, derived from re in the sense of "stream," present in place names such as Ré, in the Novara area, Redavalle and Retorbido, in the Pavia area, Redefossi, in the Milan area, or Retorto, in the Alessandria and Cremona areas. R. Brondolo confirms this hydro-toponymic origin and mentions the place name Pian del Re, placing the surname in northern Italy and the canton of Ticino, in Switzerland.

Rossoni specifies that the surname is present throughout Italy, with a zone of greater concentration in the central-western part of the north of the country. De Felice adds that the variant Lo Re is proper to the far south of the Italian peninsula.