A good OUPBlog post by Katherine Connor Martin from a few months back describes the history of derivatives of the word brother:
The English lexicon expands in innumerable ways. For instance, new words can be borrowed from other languages (café), arise through imitation of a sound (like oink or boom), and be formed from existing English words by combining two words (cupcake), blending parts of two words together (as in brunch), adding prefixes or suffixes (nationalize or unfriend), or through various types of alteration (like the respelling of phat). One way in which existing words are altered is by spelling them to represent a particular pronunciation. This might be an exaggerated pronunciation (as in puh-leeze), or a regional or colloquial one (like pardner); when these become entrenched, they sometimes cease to be the equivalents of the words they derived from, and begin to take on new meanings.
The word brother has generated a whole passel of such derivations. The OED now records at least ten distinct words based wholly or in part on regional or colloquial pronunciations of brother: bra (often spelled brah in the United States), bredda, Brer, bro, bruh, bruv, bruvver, bud, buddy, and Buh. Although they share little in common besides their first letter, all of these spellings are in some way attempts to reflect the way brother was pronounced in a particular type of speech. (Bro and buddy have slightly more complex stories, in that the former was used as a straightforward graphical abbreviation before it came to represent a regional pronunciation, and the latter may be influenced by other dialect words and by the suffix –y.) These ten words all have a connection to brother but they have developed in different ways, not all of which overlap with the meanings of the word brother itself.
Go to the link for lots of interesting details; both bro and buddy are considerably older than I would have guessed (first attested in 1533 and 1788, respectively), and “although ‘male sibling’ is the most common and familiar use of brother in contemporary English, it doesn’t occur earliest or most often among these derivatives.” And note the sly way she slips in the word passel, itself an alteration of parcel.