Когда рак на горе свистнет (Kogda rak na gore svistnet)

Literally: When a crayfish whistles on the mountain

Example: “Он вернёт долг, когда рак на горе свистнет.” — He'll pay that debt back when hell freezes over.

English equivalent: When hell freezes over

Origin: Two genuinely different origin stories compete. One traces it to an older folk proverb, first recorded in 1848, waiting "until St. George's Day, when the crayfish whistles" — tied to folk belief that a crayfish might be heard whistling specifically on that saint's feast day, an event with no more likelihood of actually happening than the arrival of the day itself producing such a sound. A second, considerably grittier theory locates the phrase's true origin in Odessa criminal slang, where it originally ran "when Rak whistles on Shkodova Hill" — Rak (literally "crayfish") being the underworld nickname of a well-known thief named Rakachinsky, with the whistle referring to a signal he was expected, implausibly, to give. Either way, the joke rests on solid biology: a crayfish has no lungs and therefore no means of whistling at all, and as a water-dwelling creature has no reason to be found on a mountainside in the first place — doubly, redundantly impossible.