Pigs might fly
Definition 1 of 1
Pro Tip 1/3Sarcasm
This phrase is often used sarcastically to express doubt about the likelihood of an event. 
You becoming an astronaut? Sure, when pigs might fly.
Pro Tip 2/3Cultural Improbability
This idiom exists in different forms in various cultures, but it always implies an impossibility. 
He'll volunteer to wash the dishes, when pigs might fly!
Pro Tip 3/3Informal Humor
Use this phrase in casual settings for a humorous effect, not in formal situations. 
Me, win the lottery? Ha, when pigs might fly!