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Jyutping nou5 seon3 dou1 mei6 saang1 maai4
Pinyin nǎo xìn dōu wèi shēng mái

Definitions (Wiktionary)
  1. phrase, Cantonese, idiomatic
    to be immature; to be naive and childish

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Jyutping nou5 seon2 dou1 mei6 saang1 maai4
Pinyin nǎo xìn dōu wèi shēng mái

Definitions (粵典–words.hk)
  1. 語句
    腦​囟​本身​都​應該​一​兩歲​就​埋口​,​用​嚟​形容​人​好似​BB​仔​咁​幼稚​、​唔​成熟
    childish, immature; literally, the soft spot (on the head) has not closed
    • 成二十歲人腦囟都未生埋,人哋講佢就信。
      成二十岁人脑囟都未生埋,人哋讲佢就信。
      seng4 ji6 sap6 seoi3 jan4 nou5 seon2 dou1 mei6 saang1 maai4, jan4 dei6 gong2 keoi5 zau6 seon3.
      He's twenty but his soft spot hasn't closed yet. (How could he) believe in everything others say.
  2. 參看
    腦囟未生埋
Examples (粵典–words.hk)
  1. Cantonese
    脑囟都未生埋
    腦囟都未生埋
    nou5 seon2 dou1 mei6 saang1 maai4
    • childish, immature; literally, the soft spot (on the head) has not closed
  2. Cantonese
    成二十岁人脑囟都未生埋,人哋讲佢就信。
    成二十歲人腦囟都未生埋,人哋講佢就信。
    seng4 ji6 sap6 seoi3 jan4 nou5 seon2 dou1 mei6 saang1 maai4, jan4 dei6 gong2 keoi5 zau6 seon3.
    • He's twenty but his soft spot hasn't closed yet. (How could he) believe in everything others say.