人心隔肚皮

人心隔肚皮

Literally means:

a man's heart is hidden from his belly

Actually means:

it's hard to read person's mind; you never know what the other person is thinking

It means it is hard to read a person's mind or to understand a person.

It is often used to indicate the speaker's disapproval of a particular person, implying that he is double dealer. Derogatory.

指人的心很难看透，或要真正了解一个人并不容易。多指对某人的不满，认为他表里不一。具有贬义色彩。