The little used positive form of innocuous. And no, it's not a later back-formation -- it's the real stem, just one that fell out of use in favor of a cousin. The root here is Latin nocuus, harmful, from nocēre, to hurt -- while the noun form, noxa, instead gave us noxious. Not to mention, the present participle of the verb's negative, innocēns, in turn gave us innocent, essentially meaning unharmed. Digging further back, nocēre in turn is close cousin to Greek nekros, a dead body, and underlying it all is the Proto-Indoeuropean root *nek-, meaning death. From such a seed, so much damage.
Perfume or cologne applied on top of body odor combines to a nocuous smell for others.
— Jodi R. R. Smith, Perfectly Polite While Working-out---L.