He told the ridiculous joke with a straight-faced expression, making everyone burst into laughter.

straight-faced 是由形容词 straight（直的、直接的）和名词 face（脸）加上后缀 -ed 组成的复合形容词，字面意思是"表情笔直的"。 straight-faced 用来形容人"板着脸的、一本正经的"，通常指在说笑话或做滑稽事情时故意保持严肃表情的样子。