Tā dāngmiàn yī tào, bèihòu yī tào, zhēnshì liǎngmiàn-sāndāo.
One way to your face, another behind your back - he's a total backstabber.
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history
First recorded in 李行道's Yuan-dynasty play 灰阑记, in which the phrase describes a scheming wife plotting against her husband. The imagery is of showing two faces while hiding three knives.
