Definition from Wiktionary, the free dictionary
|to protect; to defend; to guard (against)||man; person; people||
(literary equivalent of 的); (subor. part.); him; her; it
-less; not to have; no; none; not; to lack; un-
Literally: One must not lack a heart that is on guard.
防人之心不可無 (traditional, Pinyin 防人之心不可无), simplified
- You will not survive in life unless are on guard against those would harm you.