poker faced

/ˈpoʊkər ˌfeɪst/
adjective
  1. Having a facial expression that shows no emotion or reaction, as if hiding one's thoughts.
    • He kept a poker-faced expression while playing the final hand of the card game.
    • The detective remained poker-faced during the suspect's interrogation.
    • She was poker-faced when she heard the surprising news, not letting anyone know what she thought.