家賊 [－贼]

Jyutping gaa1 caak6
Pinyin jiā zéi

Definitions (Wiktionary)
  1. noun, Mandarin, Northeastern
    sparrow
  2. noun
    undiscovered traitor
  3. noun
    thief within a house or family
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