keep a straight face
/ˌkiːp ə ˌstreɪt ˈfeɪs/idiom
to manage not to laugh or smile
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Часто с can’t, couldn’t или hard to: I could hardly keep a straight face.
“I could hardly keep a straight face when he fell over.”
“How did you keep a straight face?”
“She told the joke while keeping a perfectly straight face.”