'Banjax' is an Irish word, in common usage (it might come from Dublin, but no-one seems all that sure about it). It can be used as a verb meaning to break, destroy, mess up or scupper; or, as 'banjaxed', as an adjective.
Terry Wogan wrote a book in 1980 called 'Banjaxed'; the word also appears in Waiting for Godot (by the bizarre but rather cool Samuel Beckett) - 'Lucky might get going all of a sudden. Then we'd be banjaxed'.
It sounds a fluffy sort of thing really - and it's not.