Ingenuous comes from a Latin term meaning of free birth, and it still appears in this sense in writing on Roman history. Elsewhere, the word means (1) frank and straightforward; and (2) naïve, artless, lacking in cunning. Ingenious means marked by great cleverness or imagination.
Ingenious is easy to remember because of its similarity to genius in sound and meaning. But despite their similarities, ingenious and genius have different sources, and ingenuity (ingenious‘s corresponding noun) is not necessarily marked by genius.
Ingenius is not a dictionary-recognized word.
Ingenuous is often misused in place of ingenious:
[O]ne of the most ingenuous and useful innovations of the 20th century: radar. [Appleton Post Crescent]
His case came to the notice of the judge through an ingenuous method: He applied for “leave to file an appeal out of time”. [Daily Nation]
These writers use ingenuous well:
In his shameless, ingenuous enthusiasm as much as his staggering productivity, Pruitt is one of Andy Warhol’s heirs … [D Magazine]
Few such clever films have felt so ingenuous and heartfelt. [Irish Times]
And here are a few examples of ingenious used well:
This simply ingenious kit is fashioned from a recycled wine bottle cut in half with the neck inverted back into the base. [Washington Post]
Remember all the steam engines, chocolate-making and other ingenious ways that our regional capital has found of earning its living. [Guardian]
Dungeons are as-usual filled with ingenious puzzles, requiring clever use of items like bombs, boomerangs and a vacuum cleaner. [Sydney Morning Herald]