Augmentative suffix akin to "large."
From Vulgar Latin *-one 'id.' Of unknown origin. It is phonologically possible that it comes from Latin -onem, the accusative of -o, a suffix forming masculine nouns, however this is semantically difficult.
If from Latin -o, then from Proto-Italic *-ōn- 'id.' From Proto-Indo-European *h3onh2- 'id.'