The phrase “when pigs fly” is one of my favourites and is a common humorous euphemism for the word “never.” It dates back to the early 1600s when more than a few writers alleged that pigs could fly with their tails pointed forward … quite an impossible feat.
The Walrus in Lewis Carroll’s 1865 book Alice’s Adventures in Wonderland pondered the possibility of ‘whether pigs had wings’ which implies an unspoken question as to whether pigs could fly hence the wings reference. It is most likely that this question happened in Mr. Carroll’s book due to the 1862 publication The Proverbs of Scotland where the following can be found:
If a pig had wings, it could fly.
Historically Speaking 