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This question already has an answer here:

Inspired by this question on English SE, and the List of idioms of improbability in Wikipedia which does not have an entry for Japanese, I was wondering... Is there an idiom in Japanese for something that will never happen?

marked as duplicate by Chocolate, snailboat Mar 19 '18 at 0:55

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This one is not strictly 'never', but it does indicate very low probability:

瓢箪から駒 (ひょうたん から こま) a horse from a gourd

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This one's in jisho.org with the definition of "when pigs fly", which is one of the English idioms that means the same thing:

百年河清【ひゃくねんかせい】を待つ -> Waiting a hundred years for the Yellow River to clear

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