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?
Please answer the following question by reasoning step-by-step.
a centuries - old Scottish proverb
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.

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Thus, the answer is a centuries - old Scottish proverb