心知肚明

xīn zhī dù míng

to know full well (without saying it)

Character by character

心xīn
heart

知zhī
knows

肚dù
belly

明míng
clear

谁对谁错，大家心知肚明。

Everyone knows perfectly well who's right and who's wrong.

Tone: neutral.

Implies an unspoken truth.

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This idiom and thousands more are in Learn Chinese with Bao, with audio, pinyin and drills that make them stick.