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'.”
(..) Can you, Mag? Want to give it a try? Come on, sound it out. Ji... immm. Jim. Pretty, but real stupid. Shut up, you fat pig! Did you honestly just say "fat pig"? You are my sister... and the best you can do is "fat pig"? Get out of my life! Oh! (..)2005 In Her Shoes