His joke was so absurd that no one in the room could keep a straight face.

keep 是动词，意思是"保持、维持"。 a 是不定冠词，表示"一个"。 straight 是形容词，意思是"直的、笔直的"，引申为"严肃的、不笑的"。 face 是名词，意思是"脸、面部"。 keep a straight face 是一个固定短语，字面意思是"保持一张严肃的脸"，实际指"忍住不笑、保持严肃表情"。