家贼难防

家贼难防

Literally means:

a thief in the family is the most difficult one to guard against

Actually means:

it is most difficult to take precaution against a bad guy from within an organization

家贼, a thief inside the family itself.

This phrase means it is difficult to guard against a thief from the family itself. It figuratively means a person of ill intention from within an organization is the most difficult to take precaution against, and the potential damage is more serious. It connotes a sense of helplessnes

家贼，家里的贼。

家贼难防，指家里的贼很难防备。比喻内部的坏人很难觉察和防范，危害更大。含有无可奈何的意味。