The expression signifies an event that is highly improbable, if not impossible, to occur. It conveys disbelief or skepticism about a proposed situation. The phrase “when pigs fly” serves as a hyperbolic statement, highlighting the unlikelihood of something happening.
The enduring popularity of this idiom lies in its concise and vivid imagery. While pinpointing the exact genesis of every figure of speech is difficult, evidence suggests usage of similar expressions dates back to the 17th century. One early instance is traced to a Scottish proverb documented in 1616: “Pigs fly in the air with their tails forward.” This demonstrates the established concept of using the absurdity of flying pigs to represent impossibility during that period.