keep a straight facev exprverbal expression: Phrase with special meaning functioning as verb--for example, "put their heads together," "come to an end."
informal (look serious, avoid smiling)
拉长脸 lā cháng liǎn
板着脸 bǎn zhe liǎn
I couldn't keep a straight face when she said she was a virgin. It was difficult to keep a straight face when I played that prank on my coworkers.