expression, godan verb (-u)
engage in a bloody feud; quarrel with one's own flesh and blood
Idiomatic expression describing violent conflict within a family or close group. The literal image is 'washing blood with blood', implying a cycle of revenge or bitter internal strife.
あの兄弟は血で血を洗う争いを続けている。
Those brothers are locked in a bloody feud.
血で血を洗うような内輪もめは避けたい。
I want to avoid a family quarrel that's like washing blood with blood.