The authors agree that this surname derives from the given name Fedele, coming from the Latin fidelis, which means "faithful". A. Bongioanni notes that it is a Latin augural name linked to several saints. Surnames in Italy specifies that Fidelis was originally a Roman cognomen which in the early times of Christianity acquired the sense of "the one who believes in the true God", and that the surname was already quite well known in the 13th century in Monza, Cremella and Bulciago.

Regarding the saint who inspired the name, the authors differ in some details. Bongioanni and E. De Stefani agree that Saint Fidelis was martyred in Como, although De Stefani identifies him as one of the patrons of that diocese. Surnames in Italy adds that he was a soldier of Maximian and that he was beheaded in the place known as vicum sumolacum, identified with Samolaco, in the province of Sondrio. Bongioanni adds that his remains were buried in Milan, in the church that bears his name, and that his feast is celebrated on 28 October.

De Stefani documents a prominent presence of the surname in the Carnia —a mountainous zone of the north of Friuli, in the province of Udine—, specifically in the frazioni of Ovaro, in particular in Clavais, where it is recorded at least from the 16th century. He also notes that, over time, the given name became a surname and that the families that bear it appear in a sporadic way throughout Italy.