The phrase "when pigs fly" is an English idiom used to describe something that will never happen or is extremely unlikely. It is a form of hyperbole called an adynaton, meaning it expresses an impossibility. The idea is that pigs cannot literally fly, so saying something will happen "when pigs fly" means it will not happen at all. This idiom dates back to at least the 1600s and may have originated from a Scottish proverb. It has been used humorously and sarcastically to scoff at over-ambitious claims or impossible events. The phrase appears in literature, including works by Lewis Carroll, where it is used to indicate something absurd or fantastical. There are many similar phrases in other languages and cultures expressing improbability, such as "when hell freezes over" in English, or "cuando los chanchos vuelen" in Spanish, which literally means the same. In summary, "when pigs fly" means "never" or "something impossible" and is used colloquially to dismiss unrealistic hopes or statements.