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'.”
(..) -Get him! -Come on! No, no! No, no, no, no! Wait! Whoa, whoa, whoa, whoa! Pig pile! -Get outta that car! -Move it! No! Please! You don't understand! Buzz is out there. (..)1995 Toy Story