The history of cool is tangled up in legend and cliché. In music, it signals distance waiting to be negotiated—the haughty allure of bossa nova, or the loneliness of Chet Baker’s voice. For women, especially singers who trip over the border between jazz and pop, cool can be a weapon of seduction, but it’s also aesthetically dangerous. Misplayed, it can easily slip from elegant to airheaded.