Passage: The idiom is apparently derived from a centuries-old Scottish proverb, though some other references to pigs flying or pigs with wings are more famous. At least one appears in the works of Lewis Carroll:\n\nQuestion: Based on this passage, where did the phrase when pigs fly come from? Let's give stream of consciousness first:
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The important part for answering this question is: The idiom is apparently derived from a centuries-old Scottish proverb, though some other references to pigs flying or pigs with wings are more famous.
So, the final answer is a centuries - old Scottish proverb.
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