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'.”
(..) Yes... yes. He must die. You've kept him alive, so he can die at the proper moment? You've been raising him like a pig for slaughter. Don't tell me now, that you've grown to care for the boy. Lily! After all this time? Always. (..)2011 Harry Potter and the Deathly Hallows: Part 2