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Jyutping hou2 zai2 m4 dong1 caai1
Pinyin hǎo zǎi wú dāng chāi

Definitions (粵典–words.hk)
  1. 語句、俚語
    喺​香港​好​流行​嘅​一句​説​話​,​指​好人​係​唔​會​做​警察​,​表示​對​警員​品格​嘅​質​疑​;​六七十年代​前​嘅​香港​警隊​貪污腐敗​至極​,​簡直​係​有​牌爛​仔​噉​,​所以​就​有​呢​個​講法
    "good men don't join the police force"; a common saying in Hong Kong expressing cynicism towards the conduct of police officers; thought to originate from the 1960's, when the police force was utterly corrupted and effectively functioned as organized thugs

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Jyutping hou2 zai2 m4 dong1 caai1
Pinyin hǎo zǐ n2 dāng chā

Definitions (CC-CANTO)
  1. Good guy would not be a policeman, because of the rogue behaviour of the old Hong Kong Police and its corruption, Hong Kong parents generally do not want their sons to be policemen
Examples (粵典–words.hk)
  1. Cantonese
    好仔唔当差
    好仔唔當差
    hou2 zai2 m4 dong1 caai1
    • "good men don't join the police"
    • a decent person will not be a policeman
    • good men don't join the police force
Examples (Wiktionary)
  1. Cantonese
    好仔唔当差。
    好仔唔當差。
    hou2 zai2 m4 dong1 caa1.
    • Good kids don't become cops.