When pigs fly

The popular story

The phrase is usually told as a timeless English folk saying with no particular author: a heavy, mud-loving farm animal is about the least aerial creature imaginable, so when pigs fly simply means never. Some retellings credit Lewis Carroll with coining it, pointing to the Duchess in Alice’s Adventures in Wonderland. Others assume it is an ancient classical tag. The popular story, in short, treats it as very old but vague – a saying that was simply always around.

That instinct is half right. The image is genuinely old. But the record is more precise, and a good deal stranger, than the modern three-word version suggests.

What the record shows

The expression belongs to a family of sayings rhetoricians call the adynaton – a deliberate appeal to an impossible event (‘when hell freezes over’, ‘at the Greek calends’) as a way of saying something will never occur. English has used flying pigs for exactly this purpose for a very long time.

The earliest datable printed form yet found is not the clipped modern quip at all. It appears in 1581, in a sprawling religious controversy translated and extended by Walter Haddon, John Foxe and James Bell, where the writer scorns an opponent’s hopes by saying they will be fulfilled only:

when pigges flye with their tayles foreward, and when S. Iames of Compostella, and our Lady of Waltsingham become man & wife

Two things stand out. First, the proverb is already being used knowingly, as a stock sarcasm – it was clearly current before 1581, not invented on the page. Second, the pigs do not merely fly; they fly tail-first. That detail is the heart of the older proverb.

The same shape recurs through the 1600s. It is listed among proverbs in editions of John Withals’s English-Latin schoolbook, A Shorte Dictionarie for Yonge Begynners (the 1616 edition reads ‘Pigs fly in the ayre with their tayles forward’), and again in John Clarke’s proverb collection Paroemiologia of 1639 (‘Pigs fly in the aire with their tailes forward’). By 1732 Thomas Fuller’s Gnomologia records the plainer comparison ‘That is as likely as to see an Hog fly.’

The modern wordings are considerably later. The sarcastic ‘might fly’ construction surfaces in the 1830s: a sporting paper in 1834 allows that ‘a pig MIGHT fly, though it is, confessedly, a damn’d unlikely BIRD’, and the line soon settles into ‘pigs might fly, but they are very unlikely birds’. Carroll’s 1865 Duchess – granting someone about as much right ‘as pigs have to fly’ – sits near the end of this evolution, popularising the image rather than originating it. The bare American form ‘when pigs fly’ is later still.

The verdict

This is a well-attested idiom with a traceable paper trail, so the honest label is attested – with one caveat worth stating plainly. What is attested is the proverb, continuously in print from 1581 onward. The specific three words ‘when pigs fly’ are a recent crystallisation of a much older saying whose original form was longer and funnier. If you are dating the idea, 1581; if you are dating the exact modern phrase, the 19th century.

The ‘tails forward’ detail also rewards attention. It is not decorative nonsense. The older proverb piles impossibility on impossibility: flight alone would be miracle enough, yet the pig is imagined doing it backwards. The absurdity is compounded on purpose, which is exactly what a good adynaton does.

What we don’t know

We do not know who first said it, or how far back the spoken proverb runs before 1581 – the 1581 text uses it as something already familiar, so an earlier oral life is certain but undatable. We also cannot credit any single incident, region or trade as the source; attempts to tie it to a specific event are guesswork. The claim, sometimes repeated, that the saying is a ‘Scottish proverb’ reflects its appearance in later proverb collections rather than any proven Scottish origin. And the precise path from the long ‘tails forward’ form to the short modern one is gradual and messy, not a clean single coinage.

Usage today

In current English the phrase is almost always a flat refusal dressed as sarcasm: a reply to a promise or prediction the speaker regards as hopeless. British speakers lean toward pigs might fly; American speakers toward when pigs fly. The tail-first version has dropped away entirely, surviving only in dictionaries of historical proverbs. The core function, though, is unchanged across four and a half centuries – a compact, vivid way of saying that is never going to happen.