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'.”
(..) his eyes rolled back in his head. Jane, shh. What was that noise? - I swear I heard something. - Yeah. That was the sound of you being a huge, disgusting pig. No, I'm serious. See? - Oh, my God. Jane. - Jane, what if he worships you? (..)1999 American Beauty