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'.”
(..) First things first, okay? Stayin' here's goofy. We gotta book up. So what do you suggest we do? Go to a hotel? We got a guy who's shot in the belly, who can't walk, bleeds like a stuck pig. And when he's awake, he screams in pain. You got an idea, spit it out. Joe could help. (..)1992 Reservoir Dogs