Stylish, fashionable, clever. This slangy term expressing admiration dates from the early 1900s as an adjective and was popularized by black jazz musicians from the late 1940s. As an interjection it was then taken up by teenagers and then by the general population. Describing the plan for an assassination, a character in James Patterson’s London Bridges (2004) exclaimed, “Not only is this smart, it is cool as hell.”