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'.”
(..) - On your feet. Pick up your sword. Hit him till he finds his feet. It seems they've run short of poachers and thieves down south. - Now they send us squealing bloody pigs. - Jon! - Again, harder. - I yield! - Enough! - He yielded. (..)2011 TvShow: Game of Thrones Title: Cripples, Bastards, and Broken Things Season: 1 Episode 4