When I check the etymology of erogenous in OED, it is mentioned that it is incorrectly formed (along with erogenic).
Etymology of erogenous from OED:
formed as erogenic adj. + -ous suffix.
Both words are incorrectly formed.
Etymology of erogenic from OED:
< Greek ἔρως sexual love + -genic comb. form: after French érogénique.
Etymonline explains as below:
"inducing erotic sensation or sexual desire," 1889, from Greek eros "sexual love" (see Eros) + -genous "producing." A slightly earlier variant was erogenic (1887), from French érogénique.
I thought it might be something to do with the suffixes -genous and -ous, but the usage of them makes sense unless I'm missing something.
-genous: word-forming element meaning "generating, producing, yielding;" see -gen + -ous. In modern formations, making adjectives corresponding to words in -gen.
-ous: word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus
-genic: word-forming element meaning "producing, pertaining to generation;" see -gen + -ic.
Etymonline mentions as inducing erotic sensation but it doesn't seem like a big leap if we say producing erotic sensation because of the suffix -genous. Although, it might require stimulation as the phrase erogenous zone suggests.
Additionally, another similar word indigenous is directly from Latin but it is formed with the suffix -ous also.
late Latin indigenus born in a country, native ( < indigena a native: see indigene adj. and n.) + -ous suffix. [OED]
After all, I'm still not sure why erogenous and erogenic are mentioned as incorrectly formed. Is it related to the suffixes or the early usage of the word?
We also have érogénique in French and erógeno in Spanish. Are they incorrectly formed also?