Poker Face
The good old poker face. If this term means nothing to you, here comes the explanation.
A poker face means showing absolutely no emotion. Whether you are holding the best hand of your life or have already said goodbye to your chips internally, your face remains calm, neutral and as expressionless as possible.
The reason is simple: your opponents should not be able to tell how good or bad your cards are. No grin, no frown, no nervous twitch. Simply nothing.