The last time I looked at anything to do with Batman it was in the early comic books and it was all "Biff" BOP" and "POW." I think in original usages, the meanings were all pretty much the same, including "Bonk." There was a tendency some decades ago to limit "bonk" to "blows to the head," which in some cases could be self-inflicted accidents, while the other terms primarily meant blows inflicted by one aggressor against another.

Various places have a tendency to pick up on a specific meaning for generic terms, and established meanings in local vernacular may lead some to believe that the term then means only the local or "cultural class" common usage.

John