a person's brother by birth.
a male person bound to another by ties of great friendship.
something usually associated with or thought to exist inseparably from another thing, quality, circumstance, etc.: Humility is often the blood brother of patience.
a male established in a close relationship with another male through the performance of a specific ritual, as the commingling of blood.
Origin: Related forms
1350–1400; Middle English
blood brotherhood, noun