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'.”
(..) - Paul, I don't want him up there. COFFEY: Boss... ...you just be quiet now. It's all right, Hal. It's all right. Hal, watch. Just watch him. Don't come near me, pig fucker. Why do you have so many scars? Who hurt you so badly? Don't hardly remember, ma'am. What's your name? John Coffey, ma'am. (..)1999 The Green Mile