Apparently it's Japanese for "double Dutch" (though that's an unfortunate equivalent, since the Dutch were in fact the Europeans whom the pre-Meiji Japanese were the mostly likely to be able to talk to in their own language, being their sole trading partners for more than two centuries prior to the arrival of Commodore Perry). The English naturally used the Dutch as the epitome of incomprehensibility because they were near neighbours and rivals. Shakespeare employed a similar idea when he had Casca coin the phrase "all Greek to me" of Cicero, while the Greeks themselves invented "barbarian" (or so I've always heard) in imitation of the babbling and incomprehensible languages of nations beyond the Greek world.
This makes me curious about "chinbunkanbun", not least because it uses the same pattern of repeated plosives as "barbarian", but also because the third kanji (漢,"kan") is typically used to refer to things of Chinese origin. It occurs in the kanji for "kanji" itself, which means "Chinese characters". It's tempting to wonder whether 珍紛漢紛 is effectively a way of saying "double Chinese".