In the Tang Dynasty (táng cháo 唐朝), there was a minister called Li Yifu who was always affable and smiling. But in his heart he was very sinister and ruthless. He constantly schemed against people he saw as possible rivals. He was called : The knife in the smile". This idiom, derived from the above story, means disguising a ruthless nature behind a pleasant appearance.

唐朝有一个大臣李义府，表面上对人温和恭顺跟人谈话总是面带微笑。但是，内心深处却十分阴险毒辣。凡是能力比他强的人，他都要设法加以打击和谋害。当时人们称他是“笑中刀”。“笑里藏刀”是由“笑中刀”转化而来。比喻人表面装得和气善良，而内心暗藏阴险毒辣。

笑里藏刀 (xiào lǐ cáng dāo)
【翻译】 Hiding a dagger behind a smile.
【释义】 形容对人外表和气，却阴险毒辣。
【例句】 李华行事笑里藏刀，他的同事都对他敬而远之。
【近义词】 口蜜腹剑、暗箭中人
【反义词】 光明正大、明火执杖