www.alphadictionary.com

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?

As to usage, dictionary.com has this to say,

Ingenious and ingenuous are now distinct from each other and are not synonyms. Ingenious means “characterized by cleverness” or “cleverly inventive,” as in contriving new explanations or methods: an ingenious device; ingenious designers. Ingenuous means “candid” or “innocent”: an ingenuous and sincere statement; a thug with the ingenuous eyes of a choirboy.

Beyond that, they neglect to mention that while they used to mean the same thing, they actually did spring from different roots and times. Ingenious came first, 1375-1425, from Middle English via Latin ingeniōsus. Ingenuous showed up much later, 1590-1600, and was taken directly from Latin ingenuus. The people who started using this form must have felt a need for it at the time, and then they became entirely distinct later.