When pigs fly

When pigs fly is an informal way to joke that you will never do something.
Example:
"Do you think you will ever work at that company again?"
Reply:
"When pigs fly!"

Pigs are heavy animals that stay close to the ground and do not fly, so when pigs fly is a time that will never come.
Example:
"Do you think your brother will be successful?"
Reply:
"When pigs fly he will."

When pigs fly is a time that will never come because pigs will never fly.
Example:
"Will you ever take her on a date?"
Reply:
"Sure: When pigs fly!"”

-- goenglish.com