Peculiarities of English, expressly for the edification of foreigners

[interprete]

No wait, both adjectives still have a noun form (ingenuity / ingenuousness), only the meanings of the nouns and of the adjectives are switched. Right?

Sorry I’m completely mixed up. Only the ingenu- "root" as survived as a noun, and the ingeni- "root" has died out. I think I’m starting to get this.

 

[Dantius]

Yeah, looks like ingenuity and ingenuousness both come from ingenuus, but the former has taken on a meaning as if it came from ingenium/ingenious.

 

[Pacifica]

What a sad thing it is when a malapropism catches on!

 

[Clemens]

Sad and not rare.