expression, ichidan verb
rain on one side only; rain while leaving the other side dry
Idiomatic expression describing a rain shower that falls on one side of a path, ridge, or area while the other side stays dry, as if divided along a horse's back.
通り雨で、馬の背を分けるように片側だけ濡れた。
A passing shower left only one side wet, as if dividing a horse's back.
The expression literally means 'to part a horse's back,' evoking the image of rain falling on one side of the horse while the other side remains dry. The exact origin is uncertain, but it is a traditional Japanese idiom.