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'.”
(..) Reminds me of my first royaI dinner party. I accidentaIIy knocked over a suit of armor... and the spear went right through the suckIing pig. AmeIia, why don't we canceI Iessons for today... and just have some fun? MlA/ Fun? You're not too busy (..)2001 The Princess Diaries