What is the meaning of idiom When pugs fly?

The idiom "When pigs fly" means something that will never happen. It's used to express that something is highly unlikely, impossible, or absurd.

Here are some examples of how the idiom is used:

* "I'll finish this project by tomorrow, when pigs fly!" (Meaning: It's highly unlikely the project will be finished tomorrow.)

* "They'll apologize for their behavior? When pigs fly!" (Meaning: It's impossible that they will apologize.)

The idiom is often used humorously to emphasize the impossibility of something.