防人之心不可无

防人之心不可无

Literally means:

one should always guard against the harm others might do to him

Actually means:

It is part of the proverb 害人之心不可有，防人之心不可无 (One should never intend to do harm to others, but should always guard against the harm others might do to him). It indicates that one should have an awareness of self-protection and should guard against the possible harm done to him by others.

来自“害人之心不可有，防人之心不可无”。指要有自我保护的意识，小心防备别人加害于自己。