expression, godan verb (-gu)
live freely in the wilderness; sleep on a stone pillow and rinse one's mouth with river water
Idiomatic expression evoking a simple, untamed life in nature. The literal image of using a stone as a pillow and river water as mouthwash is often cited as a metaphor for rustic freedom.
彼は都会を離れ、石に枕し流れに漱ぐ生活を選んだ。
He left the city and chose a life of sleeping on a stone pillow and rinsing his mouth with river water.
The exact origin is uncertain; the phrase is a classical Chinese-derived expression (漱石枕流) that entered Japanese as a set phrase describing a hermit-like existence.