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'.”
(..) Fucking Ricans think they know everything. If they knew shit, they wouldn't be Puerto Ricans. R is for Ricans P is for pigs Would you shut the fuck up for five seconds, huh? That's right. You used to be a cop. When we're working together just limit it to two eight-balls an hour. (..)2006 The Departed