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'.”
(..) It's the evil spirits. Robin of the Hood! Son of a devil worshipper! Your father died a coward... ...cursing you and squealing like a stuck pig! You'll bring no justice to your father by dying. There are your ghosts. Wind chimes. A child's toy put to good use. (..)1991 Robin Hood: Prince of Thieves