Adynaton

Adynaton is a figure of speech in the form of hyperbole taken to such extreme lengths as to insinuate a complete impossibility:

I will sooner have a beard grow in the palm of my hand than he shall get one on his cheek.

The word derives from the Greek ἀδύνατον, neuter of ἀδύνατος, "unable, impossible".

Classical and medieval usage

Adynaton was a widespread literary and rhetorical device during the Classical Period. In the Eclogue of Plutarch, there is a long list of proverbs and the first section is titled ΠΕΡΙ ΤΩΝ ΑΔΥΝΑΤΩΝ, consisting of proverbs that are built on adynaton. The adynaton form was often used for vows and covenants, such as in the 16th Epode of Horace, 25-34.
Its plural form was translated in Latin as impossibilia. A frequent usage was to refer to one highly unlikely event occurring sooner than another:
Zenobius's collection of proverbial expressions includes "to count sand" to characterize something impossible or unattainable.
However, it largely fell into disuse during the Middle Ages before undergoing a minor revival in the works of romantic poets, who would boast of the power of their love, and how it could never end.

Fiction, folklore and drama

Adynata are sometimes used within works of fiction or drama:
Impossible tasks appear often in legends and folklore, and can form elements of ballads, riddles and proverbs. Examples include: the tale of "The Spinning-Woman by the Spring",

Modern usage

Some modern adynata include:

In modern Greek: “The fly ate iron”
In Uyghur of China, “To avoid hurting my friend’s feelings, I got pregnant.”
In Bulgarian: когато цъфнат налъмите and когато върбата роди круши. koga se pokači svinja s z´´lti čehli na krusa
in Dutch: Als Pasen en Pinksteren op één dag vallen ; from a poem by Gerrit Komrij: "Eer maakt men lakens wit met inkt dan dat ik zeg wat ik thans lijden moet". "Op St. Juttemis"
In Egyptian Arabic: بكرة في المشمش
In English: When pigs fly!, and Not before Hell freezes over! and its derivative A snowball's chance in hell. When the moon turns to green cheese.
in Finnish: kun lehmät lentävät or kun lipputanko kukkii
In French: Quand les poules auront des dents, La semaine des quatre jeudis . "À la St. Glinglin" ; "Aux calendes grecques"
In German: Wenn Schweine fliegen könnten ; Wenn Ostern und Weihnachten zusammenfallen
In Hungarian: majd ha piros hó esik
in Romanian: La Paştele Cailor
In Italian: Quando gli asini voleranno.
In Latvian: Kad pūcei aste ziedēs
In Levantine Arabic: لما ينوّر الملح ,
In Malay: Tunggu kucing bertanduk.
In Malayalam: "കാക്ക മലർന്നു പറക്കും " .
In Polish: Prędzej mi kaktus na dłoni wyrośnie.
In Portuguese: quando as galinhas tiverem dentes, nem que a vaca tussa, nem que chovam canivetes, no dia de São Nunca à tarde.
In Russian: когда рак на горе свистнет.
In Serbian or Croatian: kad na vrbi rodi grožđe.
In Slovak: keď budú padať traktory or na svätého dindi
In Spanish: Cuando las vacas vuelen, instead of "las vacas" the words "los chanchos" are also used, replacing "the cows" with "the pigs" or, in Spain there could also be used Cuando las ranas críen pelo
In Sumerian: “My ox will provide milk for you!”
in Swedish: två torsdagar i veckan. It is also said as "two Sundays in the same week", but other weekdays are rarely used.
In Turkish: balık kavağa çıkınca.
In Persian: ''vaght-e gol-e ney''