The authors agree that the surname was formed from the personal name of Germanic origin Arnaldo (Arnaudo, Arnoldo), composed of the elements arn- ("eagle") and walda- ("powerful, power, government"), with the original meaning of "powerful, who dominates like an eagle" or "ruler of the eagle".

E. De Felice specifies that the name is documented in Italy from the 8th century in the Latinised forms Arnualdus and Arnaldus, and from the 12th century in that of Arnoldus. He notes that it is of Frankish tradition for the forms in -a- and German for the forms in -ò-. The element arnu- corresponds to the weak grade of aran-, that is arn-, extended with -u.

De Felice adds that the surname is widespread mainly in Liguria and partly in Piedmont in the type Arnaldi and Arnaudi; from Piedmont to the Veneto and Trentino in the type Arnoldi; and in Sicily in the type Arnao.

E. Rossoni suggests that the forms derive, directly or through various dialectal modifications, from the medieval Italo-Germanic name Arnoaldus or from the Provençal equivalent Arnaudus. He documents it in the 12th century in Brescia with a teacher Arnaudus, a disciple of Peter Abelard.

V. Blunda, for the Sicilian variant, derives it from the Germanic compound Harnu Walda. He notes that the eagle was a sacred animal of the Germanic peoples, a symbol of sovereignty and connection with the divine pantheon. He recalls Arnold of Brescia (1090–1155), a religious reformer and preacher, one of the most important figures of the movement of opposition to the temporal power of the papacy in the 12th century.