The authors agree that this surname is of Germanic origin and was formed by aphaeresis, that is, by the loss of the initial syllable or consonant of personal names such as Gilardo and the like. V. Blunda specifies that the etymon is the Germanic compound Athal Hardu, formed by athal ("noble, belonging to the aristocracy") and hardu ("hard, valiant"), with the overall sense "strong through nobility" or "valiant of noble lineage". The root athal is the same one that gave rise to modern German Adel ("nobility") and appears in numerous European personal names such as Adelardo, Adelina, Adelaide or Ethelbert. The form Ilardo reflects the loss of the initial dental consonant, characteristic of certain southern Romance adaptations.

F. Miranda places the surname mainly in Sicily, where it is present in every province, with the greatest concentration in the Palermo area —including localities such as Lascari, Cefalù, Collesano and Gratteri—, in the Enna area and in the Caltanissetta area. Miranda also notes a presence in Campania and, more scattered, in Lombardy, Piedmont, Lazio, Emilia-Romagna, Liguria and Tuscany.