"When pigs fly" is an adynaton, a way of saying that something will never happen. The phrase is often used for humorous effect, to scoff at over-ambition.

What do flying pigs symbolize?

' When pigs fly, the meaning is that something will never take place. Its usage dates back to the seventeenth century when it was readily understood that something as corpulent as a pig was among the most unlikely creatures to take to the wing.

What does the metaphor when pigs fly mean?

US, informal. —used to say that one thinks that something will never happen The train station will be renovated when pigs fly.