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'.”
(..) Kill him! That's for Frodo! And for the Shire! And that's for my old Gaffer! Stop your squealing, you dunghill rat. I'm gonna bleed you like a stuck pig. - Not if I stick you first. - Sam! Oh, Sam, I'm so sorry. Sorry for everything. Let's get you out of here. (..)2003 The Lord of the Rings: The Return of the King