Libido

Yesterday while pondering on the word libido and its feminine gender in Portuguese - quite rare for a word ending in -o -, it occurred to me that if libido had come into Portuguese through uninterrupted evolution and not been picked up by Freud in its nominative form and taken up by all other European languages, we would have libidem today. Why? Latin words ending in -o of the third declension (genitive in -is) have extended stems in all cases except nominative. Let's take origo, originis (origin): nominative origo, genitive originis, dative origini, ablative origine, accusative originem. Latin words are said to evolve into Portuguese through the accusative sans -m, that's why we have origem. Following the same reasoning, nominative libido, genitive libidinis, dative libidini, ablative libidine, accusative libidinem would have become libidem in Portuguese, belonging to the group of countless words ending in -em.