The authors agree that this surname derives from the Germanic given name compounded from willi ("will" or "desire") and helm ("helmet" or "helm"), with the general sense of "protection of the will" or "helmet of the will".

A. Bongioanni notes that the name gave rise to numerous popular regional forms which later became surnames: Lemmo and Memmo in Tuscany, Memo in Veneto, Gelmo in Lombardy. From these forms derived surnames such as Memmi and Lemmi in Tuscany, and Gelmi, Gelmetti and Gelmini in Lombardy. In Friuli-Venezia Giulia Iem is recorded, and in the Aosta Valley Vuillermin, while in Piedmont the Latinized form Vigelmus appears. Bongioanni also mentions the Venetan Bellemo and the Tuscan Bellarmino as possible variants arisen through the phonetic shift w → b.

V. Blunda highlights the figure of William II the Good (1153–1189), Norman king of Sicily, under whose reign the cathedral of Monreale was built and coexistence flourished among the Latin, Greek and Arab communities of the kingdom. This historical context explains the spread of the name —and of its form as a surname— in Sicily.