The authors agree that this surname derives from the Latin term furnus (oven). F. Miranda broadens the etymology by noting that the word forno, from the Latin furnus or fornus, is related to the sense of "splendour" or "ardour", akin to the Greek thermon ("heat, hot place"), according to Francipane. E. Rossoni documents the use of the medieval name Furnus already in 1175, in a notarial instrument of Voghera, in the Pavia area, where the mention Ego Furnus notarius sacri palacii figures, confirmed in writing in 1268.

The surname presents multiple independent clusters of origin (ceppi), arisen in different regions without direct relation between them. Rossoni identifies a presence in Piedmont —in Turin, Bairo and Agliè (Turin area), Biella and Vigliano Biellese (Biella area) and Asti— and another in Campania: Benevento, San Leucio del Sannio and Apollosa (Benevento area), and Pozzuoli and Naples.

Miranda notes that the surname extends through some fifty localities of several regions: Piedmont, Campania, Sicily, Lombardy, Lazio, Liguria, Emilia-Romagna, Veneto, Puglia and Tuscany. In Sicily it has presence in almost all the provinces, with prominent nuclei in the Syracuse, Catania, Ragusa areas and the area of Enna. Miranda also indicates that the Furnò are typical of southern Italy.