笑里藏刀 (xiào lǐ cáng dāo)
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.