Definition of "blood brother" []

• 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. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "To him, still an unmarried cadet and working for more provident men when his contemporaries were all heads of families and solid men of property, the knowledge that he was blood brother to a senior scientist was quite the most important thing that had ever happened to him, even overshadowing his recent adventures, the meaning of which he was hardly qualified to understand anyway."
• "Her first duty was to warn the blood brother that now he and El Gerente were friends and he must not tell El Gerente’s Pepita what an attractive interpreter he was obliged to employ."