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'.”
(..) - It went smooth until... - Police! Freeze! - What? - They shot Bob. - They shot him in the head. - Fucking pigs! Oh, God! - Those motherfuckers! - You morons. You're running around in ski masks trying to blow things up? (..)1999 Fight Club