noun
A male who has sworn loyalty to another male in a ceremony in which they have mingled their blood together, making them as close as brothers.
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The two men had formed a deep bond and considered each other blood brothers.
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We've been friends for so long, we're practically blood brothers.
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Me and my boy are like blood brothers, we've got each other's backs no matter what.
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Their shared experiences in the military made them blood brothers in arms.