something unexpected happening; a joke becoming reality; a horse from a gourd (lit.)
Idiomatic expression used when something surprising or unlikely occurs, especially when a casual remark or joke comes true. Literally 'a horse from a gourd', evoking the image of something large emerging from a small, unlikely place.
What I said as a joke actually came true—it was truly a case of 'a horse from a gourd'.
something impossible; something ridiculous
Used to dismiss an idea as absurd or impossible, often in the phrase 瓢箪から駒が出るような話 (a story like a horse coming out of a gourd). This sense is less common than the 'unexpected happening' sense.
That plan is as ridiculous as a horse from a gourd.