Australian colloquialism, meaning to poke
fun at. It derives from the
Wathawurung word
burag 'no, not', and was adopted into
pidgin in that sense, now obsolete. The Wathawurung lived in the area west of where
Melbourne now stands.
The first occurrence in the OED is from 1845, where borak is glossed as 'gammon, nonsense'. The altered form 'poke borax' or 'poking the borax' is only recorded from 1945, so has no real connexion with the mineral borax.
The OED also suggests a possible connexion wih 'barrack', an Australian word meaning 'support vocably (e.g. a football team)', but the English etymology makes this unlikely: it is either from the other sense of barrack, originally a soldier's tent, or bark in the sense of 'crow': either could give rise to an intermediate sense 'boast, brag'.