日防夜防，家賊難防 [－－－－－－贼难－]

Jyutping jat6 fong4 je6 fong4 gaa1 caak6 naan4 fong4
Pinyin rì fáng yè fáng ， jiā zéi nán fáng

Definitions (粵典–words.hk)
  1. 語句
    字面​指​就算​由朝​到​晚​守住​屋企​，​由​於​對​屋企​人​冇​戒心​，​始終​防​唔​到​屋企​入​面​嘅​賊​；​引申​成好​難防備​唔​老實​、​不​忠​嘅​自己​人​，​例如​穿櫃桶​底​、​內鬼​、​間諜​等​。
    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

日防夜防，家賊難防 [－－－－－－贼难－]

Jyutping jat6 fong4 je6 fong4 gaa1 caak6 naan4 fong4
Pinyin rì fáng yè fáng , jiā zéi nán fáng

Definitions (CC-CANTO)
  1. , A thief in the family is difficult to detect; the betrayer can be the one you trust
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