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'.”
(..) No. We saw it as a way of making a movie. We would've gone anywhere. On the first day of the shoot, we had a Buddhist ceremony. We had a big pig. A big feast. Let's get some fruit, some candles, some incense. Auspicious day. Once we started filming ... (..)2001 The Matrix Revisited