Idiom B1 Informal

When pigs fly

Something that will never happen; a humorous way of saying an event is impossible

Meaning

When pigs fly — Used to say that something will never happen. It is a sarcastic, humorous way of expressing that an event is utterly impossible, since pigs obviously cannot fly.

Origin & History

The phrase is an example of an adynaton — a figure of speech that uses an impossibility to mean 'never'. Pigs have long symbolised something earthbound, heavy, and utterly incapable of flight, which makes them the perfect image for an event that simply cannot occur. An early related form appears in the 1660s, and a Scottish proverb spoke of 'pigs flying in the air with their tails forward'.

The image was made even more famous in literature: Lewis Carroll referred to flying pigs in Alice's Adventures in Wonderland in 1865. In British English, the closely related form 'pigs might fly' is just as common, both expressing complete disbelief that something will ever happen.

Example Sentences

SentenceContext
He'll tidy his room when pigs fly.Doubting someone will act
"Do you think he'll apologise?" "When pigs fly!"Sarcastic reply
The bus will be on time when pigs fly.Complaining humorously
She'll lend you money when pigs fly.Expressing strong doubt
Our team will win the league when pigs fly.Pessimism about sport
I'll believe that promise when pigs fly.Distrusting a claim

How to Use It

This idiom is informal and humorous, perfect for casual conversation and light-hearted writing. It is often used as a short, sarcastic reply on its own — "When pigs fly!" — or attached to a clause to mean the action will never happen. Because it is playful, avoid it in formal or serious contexts. In British English you can swap in the variant "pigs might fly" for the same effect.

Common Mistakes

Mistakes to Avoid

He'll help when a pig flies.

He'll help when pigs fly. — The idiom keeps the plural 'pigs', not the singular.

She'll agree when the pigs fly.

She'll agree when pigs fly. — Do not add 'the'; the set form has no article.

We'll finish on time, when pigs fly next week.

We'll finish on time when pigs fly. — It means 'never', so do not attach a real future time.

Similar Idioms

Once in a blue moon Not in a million years When hell freezes over Pigs might fly All Idioms →

Practise This Idiom

Flash Cards Flip Tiles

Practice English Idioms

Use these exercises to master idioms in context:

Flip Tiles Find the Match Idioms Quiz

Frequently Asked Questions

What does "When pigs fly" mean?
"When pigs fly" means that something will never happen. It is a sarcastic, humorous way of saying an event is utterly impossible, since pigs cannot fly.
Where does the idiom "When pigs fly" come from?
It is a sarcastic figure of speech called an adynaton, which uses an impossibility to mean 'never'. Pigs have long symbolised something earthbound; a Scottish proverb spoke of 'pigs flying in the air with their tails forward', and Lewis Carroll referenced flying pigs in Alice's Adventures in Wonderland in 1865.
Can you give an example of "When pigs fly" in a sentence?
Here is an example: "He'll tidy his room when pigs fly." — used to say humorously that something is never going to happen.
Is "When pigs fly" formal or informal?
It is informal and humorous. It is common in everyday conversation and casual writing, but it is too playful for formal academic or business writing.
What CEFR level is "When pigs fly"?
This idiom is typically taught at B1 level. It is an intermediate expression that is easy to understand and appears frequently in spoken English and casual writing.
What are common mistakes with "When pigs fly"?
A common mistake is using the singular, such as 'when a pig flies'. The set form keeps the plural: 'when pigs fly'. Learners also sometimes take it literally — remember it always means 'never', not a real future time.
What idioms are similar to "When pigs fly"?
Similar idioms include: Once in a blue moon, Not in a million years, When hell freezes over, Pigs might fly. These all express that something is impossible or extremely unlikely.
How do I practise idioms like "When pigs fly"?
LexFizz's Flash Cards and Flip Tiles exercises are great for practising English idioms. Try writing your own example sentences using the idiom in context, and listen for it in films, podcasts, and conversations.
Can "When pigs fly" be used in writing?
Yes, in informal and casual writing it adds humour and colour. Avoid it in formal academic or business writing unless you are intentionally quoting or discussing the idiom itself.
Does "When pigs fly" have the same meaning in British and American English?
Yes, "When pigs fly" is understood in both British and American English with the same meaning: something that will never happen. British speakers also often say 'pigs might fly', which carries the same idea.