In an interview on a talk show, Ben Affleck once claimed that “brother” has various meanings in Boston depending on context (I wish I could find the bit on Youtube). This observation may have been exaggerated for the purposes of humor, so I wouldn’t assign it much sociolinguistic rigor. But the word definitely seems to be a common term of endearment in that area.
“Brother,” of course, has had currency in other subcultures and ethnolects, but like “wicked,” Boston uses the word in a way that seems unique to the region. Yet I can’t quite put my finger on how it’s unique. Perhaps it’s the way the word can be applied to total strangers (like a fellow passenger on a bus) that seems so unusual.
Unfortunately, like many household words with dialectally specific meanings, “brother” is a search engine black hole. So in lieu of articles or books on the subject, two plausible hypotheses come to mind:
1.) Unabbreviated “brother” was once common everywhere, but remained stronger in Boston while there was a shift toward “bro” (or other terms) elsewhere in America.
2.) “Brother” is an entirely separate development in Boston.
Lending credence to hypothesis 1 is that Boston is a strikingly conservative city, linguistically-speaking. For example, it is arguably the only major city in America where young, Caucasian English speakers are quite so resolutely non-rhotic.
Then again, that very linguistic conservatism could suggest “brother” evolving from an entirely different source. Any Bostonians care to comment?