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'.”
(..) Police brutality! I have to talk to Princess Fiona! - We warned you! - Ow! Ow! Did someone let the cat out of the bag? You capitalist pig dogs! - Catnip! - That's not mine. Find Princess Fiona! I'm a donkey! Tell her Shrek... (..)2004 Shrek 2