When pigs fly

"When pigs fly or have wings is an expression or retort that means much the same thing as 'when hell freezes over.' In other words, it emphasises something that is absolutely not going to happen. (Many different cultures have their own versions of such improbabilities. For the French, for example, it is 'when hens have teeth'.) Some sources trace the origin of 'when pigs fly' from the late 16th/early 17th century, but these claims are dubious and are not reflected in Google N-grams, which search thousands of books for citations. However, it was certainly popularised, if not coined, by Lewis Carroll in 1865 in Alice's Adventures in Wonderland: “Thinking again?” the Duchess asked ... “I’ve a right to think,” said Alice sharply ... “Just about as much right,” said the Duchess, “as pigs have to fly."" In James Goldman's 1966 play The Lion in Winter, Henry II says, ""The day those stout hearts band together is the day that pigs get wings."" Queen Eleanor replies, ""There'll be pork in the treetops come morning."""