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'.”
(..) of an acquaintance of mine. The next summer I'm captured... -He's a pigmey. ...with six others by Baschiele tribe. They trade us for pigs, shoes and beer to a very strange american man. I hear you're not so old as you looking. (..)2008 The Curious Case of Benjamin Button