家賊難防 [－贼难－]

Jyutping gaa1 caak6 naan4 fong4
Pinyin jiā zéi nán fáng

Definitions (Wiktionary)
  1. phrase, idiomatic
    it is difficult to guard against a thief within one's house; bad people from within are the most difficult to guard against
Examples (粵典–words.hk)
  1. Cantonese
    日防夜防，家贼难防
    日防夜防，家賊難防
    jat6 fong4 je6 fong4, gaa1 caak6 naan4 fong4
    • it is difficult to defend against a dishonest or treacherous insider, such as a hidden traitor and a spy; literally: one can defend night and day but it is hard to defend against a thief from one's own family