To be two-faced; to act friendly to someone's face but speak ill of them or harm them behind their back; a hypocrite.
小李总是在领导面前表现得特别勤快，背后却抱怨连连，真是当面是人背后是鬼。
Xiao Li always acts very diligent in front of the manager, but complains incessantly behind their back; he is truly two-faced.
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