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'.”
(..) I need herbs. Something to bring down his fever. I have nightshade. I have feverfew. They're no use to me. Do you have any kingsfoil? No, it's a weed. We feed it to the pigs. Pigs? Weed. Right. Don't move. You want me to find a jewel? (..)2013 The Hobbit: The Desolation of Smaug