game face
1. A figurative or literal facial expression denoting a mental attitude of determination or resolve in the face of an imminent and difficult task, activity, or workload. The thesis committee is going to be merciless, so be sure to have your game face on when you go in to defend your dissertation. I could tell by the steely look in her eyes that Janet had her game face on for the match.
2. An expression of stoicism intended to mask one's emotions or intentions. John put his game face on as soon as I started to break up with him, so I can't really tell how he took the news. The police are going to use every interrogation tactic they know to get you to confess, so be sure you've got your game face on when they come.