expression, noun
sworn friend; inseparable friend
A classical Chinese-derived expression for a friend so close one would risk one's life for them. Literary and somewhat formal; not used in casual modern conversation.
彼とは刎頸の友と呼べる間柄だ。
He and I are what you would call sworn friends.
戦場で生死を共にした二人は、まさに刎頸の友だった。
The two who faced life and death together on the battlefield were truly inseparable friends.
From the Chinese historical story of Lian Po and Lin Xiangru, where they became friends willing to die for each other. The phrase literally means 'friend of the cut neck', symbolizing a bond so strong one would not hesitate to sacrifice one's life.