The authors agree that this is a typically Sicilian surname. E. Rossoni notes that its presence is concentrated mainly in Ragusa, Comiso and Vittoria, with representation also in Palermo, Noto, Avola, Syracuse, Rosolini, Gela, Mazzarrone, Caltagirone, Calatafimi and Alcamo. According to Rossoni, the surname would derive from a nickname linked to the dialect term guastedda, which designates a type of round loaf of about a kilo, of probable Norman origin and related to the archaic French word gastelier ("pastry cook"), which would suggest that the founding ancestors practised that trade.

V. Blunda goes deeper into the etymology and proposes a Germanic origin by way of the personal name Wastel, linked to a Lombard root with the sense of "bread" or "small piece of baked dough". This root gave rise to the Old French wastel and the Middle English wastel, both with the sense of "fine white bread". Blunda further warns that the popular association of the surname with the Italian guastare ("to spoil, to damage") is a false etymology: that verb comes from the Germanic wostjan, of a different sense, and its formal resemblance to the surname is a phonetic coincidence.