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'.”
(..) - I've made... weirder shit than that. They found the blonde. The picture! The picture! Fucking pigs. - I need you to move. - I'll come out, but you guys... Get the fuck out of the booth. I've been in this cage the whole time. Come on, this way. (..)1995 Se7en