Blood brother can refer to two or more people not related by birth who have sworn loyalty to each other. This is in modern times usually done in a ceremony, known as a blood oath, where each person makes a small cut, usually on a finger, hand, or the forearm, and then the two cuts are pressed together and bound, the symbolism being that each person's blood now flows in the other participant's veins.
The act carries a risk due to blood-borne diseases. The process usually provides a participant with a heightened symbolic sense of attachment with the other participant.
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