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'.”
(..) Open your mouth and eat this. - You have to eat some food from this world, or else you'll disappear. - No! Don't worry. It won't turn you into a pig. Chew it and swallow. There you go. You're all better. See for yourself. - I'm okay. (..)2001 Spirited Away