poker face

noun
  1. 1

    An impassive facial expression cultivated to prevent other players from determining whether one's actions in the game are the result of a quality hand, or of bluffing.

  2. 2

    Any similar expression used to prevent giving away one's motives, feelings, or situation.

  3. 3

    A person who has a poker face

Source: https://en.wiktionary.org/wiki/poker%20face