Ingenuity/ingenious

[Audiendus]

Ingenuity

I am puzzled about the etymology of this word. Can anyone explain how it came to be used as the noun form of ingenious? It obviously arose from confusion with ingenuous, but how could this have happened, given that the two words are nearly opposite in meaning?

Why the illogical 'u'? Why not ingenity, ingeniosity or ingeniousness?