poker face 
noun
1An impassive expression that hides one’s true feelings.In Europe and North America, these display rules encourage vivid facial expressions of emotion; a poker face is generally regarded as dull or deceptive....
- Scott hid his grin behind a poker face - something he'd learned from Johnny.
- When we express our goodwill, we usually use a smile or a welcoming expression, not just a political poker face.
1.1A person with a poker face.
Derivatives
poker-faced
/ adjective ...- That said, the Cleveland performances were oddly disappointing: inflexible, poker-faced, and without a hint of the sensuous rubato that gives this music its infectious lilt.
- Then it was inside to start the jamming in earnest, taking up residence upstairs with Ted's father George and some of his band members, including their wonderful and absolutely poker-faced mandolin player.
- His clipped voice and poker-faced appearance point to a rather humourless, intimidating figure, but his weary stoicism and commitment to the truth make him the kind of man you would want on your side when the chips fall.
Synonyms
expressionless, inscrutable, unexpressive, inexpressive, impassive, dispassionate, blank, unresponsive, stony, wooden, empty, vacant, glazed, fixed, lifeless;
deadpan, straight-faced