What is the origin of the word cool?

Answer 1

Jazz Musician Lester Young is said to have popularised the term in the 1930's in Jazz circles with his style of "smooth Jazz" It became everyday use when after the 2nd World War the Cool Jazz movement happened.

I would question the answer above about the Jazz musician only because the word "cool" was used in the 1923 presidential campaign by the interim president Calvin Coolidge. Popularized by a slogan "Keep cool with Coolidge!".