noun
men who have had sex with the same woman; hole brothers
Vulgar slang used among men to refer to other men who have slept with the same woman. The literal meaning is 'hole brothers'. Compare with 竿姉妹 (women who have slept with the same man).
See also: 竿姉妹
あいつとは穴兄弟だよ。
He and I have slept with the same woman.
まさかお前と穴兄弟になるとは思わなかった。
I never thought I'd end up being hole brothers with you.