straight face
noun [ C usually singular ] uk/ˌstreɪt ˈfeɪs/ us/ˌstreɪt ˈfeɪs/
故意板起的面孔，绷着的脸
a serious expression on your face that you use when you do not want someone to know that you think something is funny
Blake looked ridiculous in leather trousers, and I was desperately trying to keep a straight face. 布莱克穿着皮裤看上去很可笑，我拼命忍着不笑出来。