WHEN PIGS FLY
The phrase pigs might fly or pigs may fly has been used in various forms since the 1600s, when it was said that pigs fly with their tails forward, used as a sarcastic remark something overly optimistic. The form when pigs fly is more common in America.
Meaning: Something that will never happen or something is highly unlikely to happen
1. I asked my boss if I could go on a two month vacation, he said yes, when pigs fly!
2. I think he'll pay you back your money when pigs fly.
3. I think I'll start working on my project from tomorrow. Yes, when pig fly!