A term used to describe a good friend that you have known almost your whole life. The word "brother" is used because the friend is extremely close to you to the point that he is almost like your brother--but from a different mother. And that rhymes. And you know that rhymes. Admit it!
Term commonly used by one Christian man to another. This makes sense since all Christians are "brothers" (belonging to the body of Christ) but most have different mothers.