straight做形容词解释时，是“直的，笔直的；正直的；整齐的”等意思，a straight face 板着脸；一本正经的脸。

【例句】
You have to wonder how anyone could say that seriously and with a straight face.
你一定会惊奇怎么会有人能一本正经地说出那些话。

I kept a straight face as long as I could, but I finally had to crack up.
我尽量板着面孔，但最后还是控制不住失声大笑起来。