Synchronically, comparative of magnopere (“much, greatly”), adverbial form of magnus (“big, great”), built from its root + Proto-Indo-European *-is, zero-grade of *-yōs. Full grade in maior/maius.
Diachronically from Proto-Italic *magis, from Proto-Indo-European *m̥ǵh₂-is from Proto-Indo-European *méǵh₂yōs.