straight-faced

1. ADJ A straight-faced person appears not to be amused in a funny situation. 表情严肃的; 绷着脸的

...a straight-faced, humourless character. ...一个表情严肃、毫无幽默感的人。

"Whatever gives you that idea?" she replied straight-faced. “你怎么会有那种想法呢?”她绷着脸答道。