Okay, in guise of making a cup of tea I have sneaked on tippy-toe to the library and found some answers and questions.

The word is first cited from 1845, as a definition, borak, gammon, nonsense. This suggests the author is glossing a native word. Then come various instances of 'poking borak', in quotes as a colloquialism. The first occurrence of 'borax' found is only 1945.

The OED suggests a connexion with 'barrack', but I think they're wrong. The earliest uses of 'barrack' are all in the modern sense of 'call out, support', not particularly closer to 'borak'.