笑裡藏刀

First used for describing Li Yifu:

義府貌狀溫恭，與人語必嬉怡微笑，而褊忌陰賊。既處權要，欲人附己，微忤意者，輒加傾陷。故時人言義府笑中有刀，又以其柔而害物，亦謂之「李貓」。 [Literary Chinese, trad.]

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义府貌状温恭，与人语必嬉怡微笑，而褊忌阴贼。既处权要，欲人附己，微忤意者，辄加倾陷。故时人言义府笑中有刀，又以其柔而害物，亦谓之“李猫”。 [Literary Chinese, simp.]

From: The Old Book of Tang, 945 CE

Yìfǔ màozhuàng wēngōng, yǔ rén yǔ bì xīyí wēixiào, ér biǎnjì yīnzéi. Jì chù quányào, yù rén fù jǐ, wēi wǔ yì zhě, zhé jiā qīngxiàn. Gù shírén yán yìfǔ xiào zhōng yǒu dāo, yòu yǐ qí róu ér hài wù, yì wèi zhī “Lǐ māo”. [Pinyin]

Li Yifu appeared to be mild and humble. When he was talking to others, he was always smiling. However, he was actually full of treachery and machinations. When he rose to power, he demanded people to follow him. He would set up anyone who violated his ideas. Therefore, people at that time said that he had knives in his smile, and he was also referred to as "Cat Li", referring to his insincere smiles.

笑裡藏刀

1. to act in a friendly manner with hypocritical intentions; to be a wolf in sheep's clothing

• 口蜜腹劍／口蜜腹剑 (kǒumìfùjiàn)
• 笑騎騎，放毒蛇／笑骑骑，放毒蛇 (siu³ ke⁴ ke⁴, fong³ duk⁶ se⁴) (Cantonese)
• 面浪笑呵呵，肚裡毒蛇窠／面浪笑呵呵，肚里毒蛇窠 (Wu)