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 got to go. - Where are you going? Don't start. You know I got to go do this thing. Where are my keys? Over there. It is a mess. It is like a pig pen. Why do you think I got you a dishwasher? Fucks up my nails. You got to be smart. There is enough powder here to put us away forever. (..)1990 Goodfellas
(..) What are you talking about? You mean the restaurant? It is a classy place. You've been in here a million times. Tommy taking over this joint is like putting a silk hat on a pig. I don't mean any disrespect, but that is the way it is. I'm begging you. What can I say? What am I going to do? (..)1990 Goodfellas