blood brother

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A brother by birth.
• noun One of two individuals who vow mutual fidelity and trust by a ceremony involving the mingling of each other's blood.

from The Century Dictionary.

• noun A brother by birth.
• noun One who has become a brother by the ceremony of mutual mingling of blood, as among the American Indians and others.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A brother by birth.
• noun Someone bound to another person in commitment and friendship by a ceremonial mingling of blood.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a male sworn (usually by a ceremony involving the mingling of blood) to treat another as his brother
• noun a male with the same parents as someone else