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'.”
(..) He was four years old. He never used to cry. He just used to sit next to me and cuddle. I was more of a mother to him than that goddamned pig ever was. Don't talk like that about pigs. They're usually nicer than people. But they smell like shit. Not true. As a matter of fact, right now I have one in my kitchen... that's very clean... - and smells very nice. - You don't have a pig in your kitchen. Yes, I do. I was just in there, and I didn't see any goddamned pig. Don't move. I'll get him. (..)1994 Léon: The Professional