A flying pig is a symbol of an impossible event coming to pass. The popular vernacular, saying something will happen "when pigs fly", or "when pigs have wings" is traditionally used to mean that the specified event will never occur.
Rises from the general belief that hell is an extremely hot place. Expressions include "when hell freezes over", "a cold day in hell", "Satan will be ice-skating to work", and "a snowball's chance in hell".
Don't hold your breath
Used to indicate that something hoped for will likely never happen, implying that if one held their breath while waiting, they would die.
Arabic has a wide range of idioms differing from a region to another. In some Arab countries of the Persian Gulf, one would say إذا حجت البقرة على قرونها ("when the cow goes on pilgrimage on its horns"). In Egypt, one says في المشمش, ("when the apricots bloom"). Other Arab people, mainly Palestinian, use the expression لما ينور الملح, which roughly translates into "when salt glows", which means "never" because salt is not a glowing material.
Chinese – 太陽從西邊升起 ("when the sun rises in the West")
Czech language – Až naprší a uschne meaning "Till the rain will not dry". Another expression is Až opadá listí z dubu ("Till the leaves are not going to fall from oak")
Dutch – met Sint-juttemis (nl), or als Pinksteren en Pasen op één dag vallen ("when Pentecost and Easter are on the same day")
Esperanto – je la tago de Sankta Neniam ("on Saint Never's Day")
Finnish – sitten kun lehmät lentävät - when the cows fly. Also jos lehmällä olisi siivet, se lentäisi (if cow had wings, it would fly), implying futile speculations. Also kun lipputanko kukkii ("when flagpole blossoms") and Tuohikuussa Pukin-päivän aikaan ("in Barkember on St. Buck's day", implying an imaginary month and imaginary day). Sometimes also kun Helvetti jäätyy ("when Hell freezes over"), although saying it aloud to someone is considered very rude and hostile.
French – à la Saint-Glinglin (on Saint Glinglin's day). Glinglin is a nonsense rhyme for the French word saint. Another expression is la semaine des quatres jeudis ("the week of the four Thursdays"), as in "that will happen (or not) during the week of the four Thursdays" (Thursday was the break in the school week). Another expression is quand les poules auront des dents ("when hens have teeth"). The expression aux calendes grecques ("to the Greek Calends") is also used for indefinite postponement, derived from the ancient Latin expression (see below).
German – Wenn Schweine fliegen können! is identical with the English saying "when pigs fly", although the older proverb Wenn Schweine Flügel hätten, wäre alles möglich ("if pigs had wings, everything would be possible") is in more common use, often modified on the second part to something impossible, like "if pigs had wings, even your idea might work". Another phrase is Am Sankt-Nimmerleins-Tag ("on St. Never's Day").
Georgian– როცა ვირი ხეზე ავა ("when the donkey climbs the tree")
Greek – Του Αγίου Ποτέ ("on St. Never's [Day]") is sometimes used, however the profane Του Αγίου Πούτσου ανήμερα ("right on the Day of St. Dick's") is more popular. A common expression used to point out someone's wishful thinking is Αν η γιαγιά μου είχε καρούλια, θα ήταν πατίνι ("If my grandmother had wheels she would be a skateboard").
Hebrew – כשיצמחו שיערות על כף ידי ("when hair grows on the palm of my hand", a part of the human body where hair never grows). Another is a legal term, referring to the indefinite postponing of a case, "until Elijah comes".
Hungarian – The two most often used expressions are majd ha piros hó esik ("when it's snowing red snowflakes"), and majd ha cigánygyerekek potyognak az égből ("When gypsy children are streaming from the sky"). There is a third one: majd ha fagy ("When it freezes"), the short version of majd ha a pokol befagy ("When hell freezes over"), which is not used much anymore. Another expression is holnapután kiskedden ("on the less holy Tuesday after tomorrow").
Italian – Common idioms are quando gli asini voleranno ("when donkeys will fly") and, similarly to Latin, alle Calende greche ("to the Greek Kalends"). To imply futile speculations, a common expression is e se mio nonno aveva le ruote, era una carriola ("if my grandpa had wheels, he'd be a wheelbarrow").
Latin – ad kalendas graecas ("to the Greek Kalends") signified indefinite postponement, since the Greek calendar had no Calends period
Limburgish – Te Pruimpaschen, als de kalveren op 't ijs dansen (usually shortened to just met Sint-juttemis[clarification needed]), meaning "on Saint-juttemis day, when the calves are dancing on the ice". This Saint-juttemis is usually thought to be a fictional saint in the vein of Saint Glinglin but is in fact real (St. Judith in English). Therefore, Sint-juttemis day is an actual day (the 17th of August) and this makes saying just "on Saint-juttemis day" when one means "never", a prime example of irony. The adding of the phrase "when the calves are dancing on the ice" is what makes the phrase impossible, because it never freezes on the 17th of August in the Netherlands.
Lombard (Milanese dialect) – quand pìssen i òch ("when the geese will piss"), refers to the popular belief that a goose never urinates
Malay – menunggu kucing bertanduk ("to wait until a horned cat walks by")
Malayalam – കാക്ക മലർന്നു പറക്കും (kākka malarnnu paṟakkuṃ), "[the] crow will fly upside down"
Marathi – आत्याबाईं ना मिश्या असत्या तर काका म्हंटले असते (Ãtyābāiḥ nā mishyā asatyā tar kākã mhaṭalā asatā), "if aunt (here: father's sister) grows moustaches she would be called uncle"
Persian – وقت گل نی (vaght e gol e ney), "when the reed plant blossoms"
Piedmontese (Turin dialect) – An unlikely event will occur in the smana dij tre giòbia (the "week with three Thursdays").
Polish – na święty Nigdy ("till St. Never's Day"); zobaczysz... jak świnia niebo ("you'll see [something]] like a pig will see the heaven"); prędzej mi kaktus na dłoni wyrośnie ("sooner will a cactus grow on my palm"); (pulling down the lower eyelid of an eye) Jedzie mi tu pociąg? ("Is a train running here on me?").
Portuguese – no dia de São Nunca ("on Saint Never's day"), nem que a vaca tussa ("not even if the cow coughs"), quando os porcos voarem ("when pigs fly") and quando as galinhas tiverem dentes ("when chickens have teeth").
Romanian – la paștele cailor/la Ispas ("on the horses' Easter/on Ispas"), când o face plopul pere și răchita micșunele ("when poplars would grow pears and willows wallflowers"), la sfântul așteaptă ("on Saint Waits' Day"), and când va zbura porcul ("when pigs fly")
Russian – когда рак на горе свистнет (kogdá rak na goré svístnet), "when the crawfish whistles on the mountain"
Serbian – кад на врби роди грожђе (kad na vrbi rodi grožđe), "when willow bears grapes". Another variant is кад на врби засврби (kad na vrbi zasvrbi), "when willow get itchy". Note rhyme in vrbi zasvrbi. Мало сутра (malo sutra), literally "a little bit tomorrow", has a similar meaning as "all my eye".
Seychellois Creole, also known as Kreol or Seselwa (creole spoken in Seychelles) – lannen de mil zanmen is used, which means "year two thousand and never". It is a fairly new expression used mainly among the youth.
Slovene – Ob svetem Nikoli is a wordplay that literally means "on St. Nicholas' feast day". The word nikoli, when stressed on the second syllable, means "never", when stressed on the first it is the locative case of Nikola, i.e. Nicholas
Spanish – cuando las vacas vuelen ("when cows fly") or cuando los chanchos vuelen ("when pigs fly"). Its most common use is in response to an affirmative statement, for example "I saw Mrs. Smith exercising, I swear!" to which the response given would be something like, "Yeah right, and cows fly". Other variations slightly fallen into disuse include cuando las ranas crien pelo ("when frogs grow hair") and cuando San Juan agache el dedo ("when Saint John bends his finger"). The latter is a reference to the common depiction of St. John with one or two extended fingers.
Tagalog – kapag namuti ang uwak, kapag nangitim ang tagak ("when the crow turns white, when the egret turns black"). Note the euphony between the nouns uwak and tagak.
Turkish – balık kavağa çıktığında ("when the fish climbs the poplar tree"). Another one is çıkmaz ayın son Çarşambasında ("at the last Wednesday of the endless month")
Ukrainian – коли рак на горі свисне ("koly rak no hori svysne"), "when the crawfish whistles on the mountain"; or a longer variant коли рак на горі свисне, а риба заспіває (koly rak no hori svysne, a ryba zaspivaye), "when the crawfish whistles on the mountain and fish sings". Another expression is не бачити тобі ... як своїх вух ("you'll never see [something] like you will never see your ears").