xiào bǐ hé qīng
形容态度严肃,难见笑容。
《宋史 包拯传》:“立朝刚毅,贵戚宦官为之敛手,闻者皆惮之。人以包拯笑比黄河清。”
笑比河清偏正式;作谓语、宾语、补语;形容态度严肃。
To make somebody smile is as difficult as to purify the river.