In 1942, Paramount Studios attempted to float a new term for rising star Alan Ladd: "the romantic heavy." At the time, the "heavy" meant the villain and the romantic lead was always the hero -- so this is an oxymoron. It might be translated as the heroic badass and it didn't take. The romantic heavy was a rogue figure we now call cool, even if no one used the term at the time.
The aesthetic of noir cool was established in seven films released between 1940 and 1942, including The Maltese Falcon and Citizen Kane. These films defined the genre’s aesthetics, thematics, visual style, and moral ambiguity. Through Humphrey Bogart and later Robert Mitchum, the genre created a new mode of rebellious individual masculinity: noir cool -- a public mask of stylish stoicism.
The crucible of this new figure was his masking of emotion -- his cool. To be "cool" suggested self-control to the point of detachment, and signified an insolent defiance as registered in facial expression and body language. The masking of emotion communicates an inner intensity critical to the self-presentation and embodiment of Bogart, Mitchum and Alan Ladd.