Related Clues:

• Hard to rattle
• Showing little emotion
• Not easily flapped
• Like poker faces
• Betraying no emotion
• Giving nothing away, in a way
• Uncomplaining sort
• Follower of the philosopher Epictetus
• Having a stiff upper lip
• Not overly emotional
• Not moved much
• Not reacting to pain, say
• Hard to stir
• Epictetus, for one
• Marcus Aurelius was one