My early acquaintance with the word was when I first worked with professional musicians. They used the word "cool" when a musician had done a difficult, perfectly executed solo without breaking a sweat.
Subsequently I heard "cool" used for performances that went especially well. I'm talking the early 1940s. I was particularly pleased when a jazz group, appearing after my performance in a variety show, told me my "Bell Song" from Lakme was "Cool!"
Perhaps it was used before that but that's when I heard it.
wow