Cool Originated (1950s) West Coast. Describes a
restrained intellectual, rather than emotional,
approach to music.
Ex: Stan Getz plays some real "cool" jazz.
Now a widely used superlative.
Ex: He's one "cool" cat.
Ahem.
See the Miles Davis album, "Birth of the Cool".
The term was actually lifted from the use of Air Conditioning becoming more prominent after WWII.
The stores, restaurants and bars of the day would feature a penguin with the slogan, "Come on in, its COOL inside!" - a promotional typically installed by the air conditioning installers at the same time, this signage was ubiquitous.
Along with Miles, Gil Evans and his wonderful arrangements on "Birth of the Cool" gets the credit for this laid back style that a musician must first master bebop to truly be able to play. "Swing hard at low volume" was the catchphrase of the day. Not. Easy. To. Accomplish. "Less is more" might also describe The Cool.
The term got ripped off by the Beat Generation and came to mean something entirely different, actually.
--Mac