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'.”
(..) (All) To making it count. Bravo ! (Molly laughing) Mr Brown had no idea l'd hidden the money in the stove ! So he comes home drunk as a pig and he lights a fire ! Next it'll be brandies in the smoking room. Join me in a brandy, gentlemen ? Now they retreat (..)1997 Titanic