poker face

a poker face (a face lacking any interpretable expression; a visage lacking any expression that can be interpreted as that of an expert poker player; expressionless face, which conceals one’s feelings; an expression on one's face that does not show what they are thinking or feeling) — бесстрастное, непроницаемое лицо или выражение лица; невозмутимый, бесстрастный вид; каменное лицо, маска

have / keep / maintain a poker face (to show no emotion at all) — сохранять бесстрастный, невозмутимый вид; сохранять непроницаемое выражение лица

Example 1: He maintained a poker face.
(OALD)

Example 2: She just sat there with her straight, unblinking poker face, so you couldn’t really say whether she was happy or sad.

Example 3: In spite of the repeated details of the events, the criminal kept a poker face. (learn-english-today.com)

Example 4: Whenever Betty attended one of her children's performances, she managed to keep a poker face. (The American Heritage® Dictionary of Idioms by Christine Ammer)

=>
poker-faced {before noun} (showing no expression on your face; not showing any emotion in your face ) — с непроницаемым выражением лица; с бесстрастным лицом, видом; с непроницаемым, каменным лицом

Example : Our passports were checked by poker-faced officials. (OALD)

Cf:
keep a straight face (if you keep a straight face, you look serious although you really want to laugh) — сохранить невозмутимое, серьезное выражение лица (сдерживая улыбку или смех); не рассмеяться, сдержаться от смеха

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This term alludes to the facial expression of a poker player who is expert at concealing his feelings about his hand.
From the card-game of poker, where one shows no facial expression that could suggest whether one has been dealt a good or a bad hand.

Related vocabulary
[put on a brave face]