straight face

n. 不显现表情的脸

He has such a strange voice that it's difficult to keep a straight face when he's talking.

他的声音很古怪, 说起话来很难让人不笑.

My father never dotes on any of us. What he gives us is always a straight face.

父亲谁也不宠爱。他给我们的总是一副严肃的面孔。