The meaning
>A humourous, sarcastic remark, used to indicate the unlikeliness of some event or to mock the credulity of others.
– I might make a start on papering the bedroom tomorrow.
– Yes, and pigs might fly!
Similar phrases in English include «when hell freezes over», the Latin expression «to the Greek calends».
Etymology
> We have to go back a long way to find the original of this idea. It seems to have been a traditional Scottish proverb, which was first written down in 1586 in an edition of John Withal’s English-Latin dictionary for children. This had an appendix of proverbs rendered into Latin, of which one was the usual form of the proverb in the 16th and 17th centuries: “pigs fly in the air with their tails forward”.
If they did indeed fly, the proverb argues, flying backwards would seem a small extra feat.
Another version is more famous, because it appears in Alice’s Adventures in Wonderland by Lewis Carroll:
«I’ve a right to think, said Alice sharply…
“Just about as much right,” said the Duchess, “as pigs have to fly.”
Other forms that have appeared at various times include «and pigs could fly if they had wings», and «pigs may fly, but they are very unlikely birds».
Wrong translation
> Si les cochons volaient
Correct French idiom
> Quand les poules auront des dents
