E. De Stefani and E. Rossoni agree that this surname derives from the Germanic given name Rainer (Ranieri), composed of the elements ragin ("counsel," or in the plural, "gods") and hari ("army"), with the approximate sense of "one who gives good counsel to the army." Both note that the surname took root in the Udine area, specifically in the localities of Amaro and Tolmezzo, in Friuli.

De Stefani specifies that the old form of the surname was Rainîs, in which the Latin Rainisius is recognized, in the same way that Aloisius derived into Alvìse. According to this author, the name Rainerius was very diffused in the medieval period and in time became a surname. He places the family origin in Amaro, in the Carnia —a mountainous zone of northern Friuli—, where the surname is documented from at least the 15th century.

Rossoni, on the other hand, proposes that the surname form could derive from a diminutive variant of Slavic origin on the Germanic base of Rainer, and points to Tolmezzo as the possible place of origin of the family, where there was an old mill of that name. He further adds the existence of a nucleus in the Istrian area, in the localities of Capodistria and Cittanova (Novigrad).