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'.”
(..) - Yeah. - Who sent you? It was Joey Zasa. Are you okay? - You were going to let them kill me! - No, I wasn't... You pig! You sick fuck! You wanted gambling. That's gambling. Call the police. - You killed the other guy too? - He went for my gun. It was self-defence. Go ahead, call the police. (..)1990 The Godfather Part III