(Hyoutan kara koma ga deru; “A horse from a gourd”)
Used to describe something that is completely unexpected, or something that was suggested as possible at least half-jokingly, but which actually comes to pass. Note that this can’t be merely unlikely, like the weather being sunny despite a forecast of rain; it has to be something almost unthinkable, like a rain of fish… or like a horse passing through the tiny mouth of a bottle gourd.
瓢箪 (hyoutan) is a gourd, specifically the “bottle gourd.” から (kara) is a particle serving the function of the preposition “from” or “out of.” 駒 (koma) is a mildly archaic character meaning “horse,” although in modern usage it can also refer to a game piece (especially in shogi, also known as “Japanese chess”) or the bridge of a stringed instrument. が (ga) is the subject marker for the intransitive verb 出る (deru, “to go/come out”).
This kotowaza was selected from the Kyoto Iroha Karuta set. A shortened version, 瓢箪から駒 (verb elided) may also be used. The “joke coming true” meaning ties this phrase to ideas of surprises in reality making lies into truth, although it can be also used in its more basic meaning of an almost-unforseeable turn of events.
(“Ichiban se no hikui senshu ga dankushotto no konkuuru de kattara omoshiroi to itta kedo, masaka sou naru nante omowanakatta. Hyoutan kara koma tte konna koto da yo ne.”)
[“Yes, I said that it would be funny if the shortest person on the team won the dunking competition… but I never thought it would actually happen. This must be what they mean when they say ‘a horse from a gourd‘!”]