家贼难防[ jiā zéi nán fáng ]

⒈ 家属或内部的人作弊营私难以防范。英thief in the family is difficult to detect; a thief from within is hard to guard against; it is difficult to forestall a thief within the house;

⒈ 家庭内部的贼人或内奸最难防范。

引《五灯会元·同安志禅师法嗣·梁山缘观禅师》：“问：‘家贼难防时如何？’师曰：‘识得不为寃。’”清 李渔 《凰求凤·悟奸》：“这等看起来，真箇是家贼难防，连星相医卜的话都是他教导的了？”