keep a straight face
面无表情；保持严肃的表情
口语·描述忍住笑或保持严肃
英文释义
to maintain a serious or composed expression, especially in a humorous or difficult situation
例句
It was so funny, but I managed to keep a straight face.
典故来源
源自戏剧表演中保持面无表情以掩饰真实情绪，straight face即无表情的脸。