straight face

 绷着脸

►含义：

an emotionless facial expression

没有表情的脸

►例句：

I thought he was kidding when he told me his ridiculous plan, but he had a straight face the whole time.

当他跟我说起他那愚不可及的计划时，我以为他在开玩笑，但是他全程都是绷着脸的呀。

►对话：

A: So what exactly did you say to my father? He has kept that straight face. 

所以你到底跟我爸说了啥，他现在一直绷着脸。

B: I just made a few jokes, but I guess I need to explain to him right now.

我就是开了几句玩笑，不过我觉得我现在得向他解释一下了。

►额外收获： 

ridiculous: adj.荒唐的、可笑的

make jokes：开玩笑