auctor

Auctor, auctoris, m. Third declension, from augeo, to increase, so the root idea is the one who makes a thing grow: the founder, the instigator, the person standing behind it and guaranteeing it.

That explains the ablative absolute me auctore, which means "on my proposal, with my backing" and has nothing to do with writing anything. The literary sense is real but secondary in classical Latin and only became the main one later, which is why Cicero's auctor is more often a sponsor than a novelist. There is a feminine auctrix, but it is rare and auctor is applied to women unchanged. The medieval spelling autor lost the c, and so did French auteur and English author.