Bié jiǎxīngxīng de le, wǒ zhīdào nǐ bùshì zhēnxīn de.
Stop being phony - I know you're not sincere.
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假惺惺 is colloquial and carries a strong negative tone - it implies you see right through someone's fake sympathy or concern. More biting than simply saying 虚伪 (hypocritical).
