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 men like A.R. Lowenthal. Hear, hear. In Lithuania, when he was ... he stuck a knife in the back of a Cossack pig... who had knocked his mother to the ground. At , he prowled New York as a bandit... until he had the cash to feed (..)1974 The Gambler