This phrase is used presumably due to the unlikelihood that pigs will ever evolve wings.
"when-pigs-fly." YourDictionary, n.d. Web. 06 December 2018. <https://www.yourdictionary.com/when-pigs-fly>.
when-pigs-fly. (n.d.). Retrieved December 06th, 2018, from https://www.yourdictionary.com/when-pigs-fly
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