"Nonchalant"

       Somehow the word was uttered in an appropriate context between my son and me last night, and I simply wondered where it came from.  No ready root came to our minds.

        A good time for the dictionary.  You're right if you thought a French origin is involved.  Chalant  meaning "warm" coming from Latin way back calere, "heated" or "hot."  So "not warm or heated."  And "cool" is even a synonym.