He is a sexist 'pig'. = He is a 'male chauvinist pig'. [=he is a man who thinks women are not equal to men]
The train station will be renovated 'when pigs fly'. [=it will never be renovated]
“This time I think he'll ask me to marry him!” “Yeah, and 'pigs might fly'.”
(..) See? That's the ticket. Nice and easy. Now we can ignore each other in peace. I go, "What you doing?" Know what he tells me? He goes, "It's better than the pig I just tried to fuck." MAN : Never lets me finish. MAN : Why don't you tell a joke? MAN : Did you fuck him too? (..)1998 American History X