As far as English is concerned, the root of curiosity is curiosity. It was not formed from curious. The real question here is where the latter got its second u from.

Understanding the Context

The French original did not have it. (Edit: and the answer to that, of course, is rather boring: by analogy with all the other -ous words. Dangerous, numerous, devious, perilous, dubious, serious, oblivious, murderous, hilarious ... What is the meaning of "out of curiosity"?

Key Insights

Could it mean out of ideas? Or maybe it means he is curious? How should I know what he means? In my experience, someone who asks things out of " morbid curiosity " generally seems to do so for the "sake of fulfilling their curiosity" with no other motivation beyond boredom, a random thought (lol ADHD), and so on. However looking it up, the term seems to refer with an obsession with morbid topics, or anything generally negative, about death and disease, etc.

Final Thoughts

That said, if I say that I ... I remember hearing once a three-part expression but have not been able to remember or find the third part in years. Curiosity killed the cat. Satisfaction brought him back. What is the third? I found some ideas from the definition of incurious: Lacking intellectual inquisitiveness or natural curiosity; uninterested.

But these tend to be opposites of more common words. Are there any unique words to describe the quality of a pronounced absence of intellectual curiosity? What is the difference between the words curiosity and curiousness? How and when do you use them?