The authors agree that the surname derives from the medieval name of Germanic origin and Frankish tradition Mainardo, documented from the 10th century in the Latin forms Maginardus, Maghinardus and Mainardus. It is compounded from the elements magin ("strength, power") and hard or hardhu ("hard, strong, valiant"), with the original meaning "hard, formidable through his strength".

E. De Felice notes that the surname is distributed with medium frequency in northern Italy and sporadically in the centre. E. Rossoni specifies that the shift from magin to main is a phenomenon both Lombard and Frankish, and supplies early historical testimony: the Benedictine scholar Jean Mabillon recorded references to the surname as early as the 5th century, with a count Mainardo and a bishop of Le Mans named Maginardi.

V. Blunda documents a Sicilian presence of the surname, likewise of Germanic root. He adds that in the 12th century the Cistercian abbot Maynard of Pontigny sheltered Archbishop Thomas Becket during his exile, an episode linked to the principle of monastic right of asylum. Blunda also notes that the variant Manardo reflects the simplification of the consonant cluster proper to medieval southern Italian.