blood brother

Noun

broadly

An unrelated male considered to share a brother-like relationship established by a ceremonial sharing of blood in some cultures.

Japanese Meaning

血縁関係はないものの、ある儀式（血を分かち合う等）により兄弟のような絆が形成された男性、すなわち『血の兄弟』と呼ばれる関係

After the ritual, he swore to watch over me for life as my blood brother.

儀式の後、彼は生涯私を守ると誓い、私の血の盟約で結ばれた兄弟になった。

Sense（1）

An unrelated male considered to share a brother-like relationship established by a ceremonial sharing of blood in some cultures.

Sense（2）

(by extension) An unrelated male considered to share such a strong relationship generally.

Sense（3）

A brother by blood: a brother-german or half-brother, as distinguished from step-brothers.

blood brothers

（ plural ）

blood brethren

（ obsolete plural ）