Meaning of blood brother

blood' broth'er

Pronunciation: [key]
  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.