A large shite from a non-descript large mammal.
From the idea that a bear, should it lay an egg, would be brown, hairy and quite large.
Oh fuck it. Sorry Your Majesty, I just stepped on a bear's egg. Do excuse me for a short while whilst I wipe it from my trouser leg.
A jovial form of insult, only recently available due to legal/social attitudes.
Also can be used to someone controlling over any area of life
"Oi you theivin' nazi bastard"
"Health Nazi" (J Gaunt, 2008)
"Hello you daft nazi fool" (to be said in germanic accent)