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'.”
(..) He's got to obey. l mean, he just doesn't have anything else to do. Obey. And it's a funny-sounding word, obey. Sounds like pig Latin. Obey. Obey. Oh, yea, obey. Anyway, my wife is the boss and as l said before, l don't want her job, (..)1983 Bill Cosby: Himself