知人知面不知心

zhīrénzhīmiànbùzhīxīn ㄓ ㄖㄣˊ ㄓ ㄇㄧㄢˋ ㄅㄨˋ ㄓ ㄒㄧㄣ

    • one may know a person for a long time without understanding his true nature (idiom)

Character by character

Saying俗语俗語

Proverb谚语諺語 · In everyday use

Literally
you know a person's face but not their heart
Meaning
It's easy to know how someone looks but hard to know what they think. A warning not to trust people too quickly.

Sources