The only free and comprehensive online etymological dictionary of the Spanish language
nona f. (Noun) "grandma"
13th cent. Borrowed from Latin nonna "old woman," "wet nurse." Also note 12th cent. Old Spanish nana "married woman."
French nonne, Italian nonna

Despite the semantic argument that the word is native to Spanish, and not borrowed from Latin, the word did not undergo palatalization. Fascinatingly, a cousin of the word is ñoño, which palatalized twice - generally against the norm in Spanish.