1. a person's brother by birth.
2. a male person bound to another by ties of great friendship.
3. something usually associated with or thought to exist inseparably from another thing, quality, circumstance, etc.: Humility is often the blood brother of patience.
4. a male established in a close relationship with another male through the performance of a specific ritual, as the commingling of blood.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.