List of idioms of improbability

There are many idioms of improbability, used to denote that something is impossible or unlikely to occur.

;Flying pig "(main article)": A flying pig is a symbol of an impossible event coming to pass. The popular saying " [it will happen] when pigs fly" (or when pigs have wings) is traditionally used to mean that the specified event will never occur.;Cold hell ("article"): Rises from the general belief that hell is an extremely hot place. Expressions include "A cold day in hell", "When hell freezes over" and "A snowball's chance in hell."

In other languages

*Latin had the expression "ad kalendas graecas" (to the Greek Kalends) for indefinite postponement. The Greek calendar had no calends.
*In French, the expression is "à la Saint-Glinglin" (on Saint Glinglin's day). "Glinglin" is a nonsense rhyme for "saint". 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.
*In Portuguese the equivalent expression is "no dia de São Nunca à tarde" ("in the afternoon of Saint Never's day").
*In Spanish, something that will never happen is often referred to as "cuando las vacas vuelen" ("when cows 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 one is a reference to the common depiction of St. John with one or two extended fingers.
*In 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 edited on the second part to something impossible, like "if pigs had wings, even your idea might work".
*In Dutch, the expression is "met Sint-Juttemis, als de kalveren op het ijs dansen" (usually shortened to just "met Sint-Juttemis"), 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. Therefore Saint Juttemis day is an actual day (the 17th of August) and this makes saying just "on Saint Juttemis day" when you mean 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. Another commonly used phrase in Dutch is "als Pasen en Pinksteren op één dag vallen" meaning "when Easter and Pentecost are celebrated on the same day".
*In Russian, the expression is "когда рак на горе свистнет" (kogdá rak na goré svístnet) - "when the crawfish whistles on the mountain".
*In Finnish, the expression is "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.
*In Persian, the expression is "وقت گل نی" (vaght e gol e ney) - "when the reed plant blossoms".
*Black swan theory, a term developed by Nassim Taleb to label unexpected, rare events
*The Black Swan (book), Taleb's book on the theory.

References