Maybe a veiled reference to former Aussie PM John Curtin, who led Australia during WW2. ;-P (Although given how literate both Aziraphale and Crowley are, I doubt it...)
Or, perhaps, it's an equally unlikely misspelling that was supposed to be "Nyet, ruck!" as in "No to rucksacks" - but there were simply not enough letters on the license plate. (Again, I really doubt it). ;-P
Seriously, though, I have no idea. Now I'm curious myself.