Related Clues:

• Dated expression of appreciation
• 'Cool!' (Sl.)
• Seneca, for one
• Unemotional thinker
• Rapper Mos __
• Car part, slangily
• Devoid of emotion
• Hard to rattle
• Badass
• Physicist's measure
• "Really cool!"
• Showing little emotion
• Unit at a reactor
• Not easily flapped
• OED offering