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Jyutping jyu4 bat1 ho2 kap6
Pinyin yú bù kě jí

Definitions (CC-CEDICT)
  1. impossibly stupid
Definitions (粵典–words.hk)
  1. 形容詞
    形容​蠢到​冇​人​比得上
    hopelessly stupid; height of folly; literally: could not be more foolish
    • 用油嚟滅火?你真係愚不可及。
      用油嚟灭火?你真系愚不可及。
      jung6 jau4 lai4 mit6 fo2? nei5 zan1 hai6 jyu4 bat1 ho2 kap6.
      Who would put out a fire with oil? How stupid you are!
Definitions (Wiktionary)
  1. adj, idiomatic
    very stupid; stupid beyond reach
Examples (粵典–words.hk)
  1. Cantonese
    用油嚟灭火?你真系愚不可及。
    用油嚟滅火?你真係愚不可及。
    jung6 jau4 lai4 mit6 fo2? nei5 zan1 hai6 jyu4 bat1 ho2 kap6.
    • Who would put out a fire with oil? How stupid you are!
Examples (Wiktionary)
  1. Mandarin
    愚不可及
    愚不可及
    yú bù kě jí
    • inimitable foolishness