The term is coined from an old English expression and is used to describe someone who is homosexual.
It comes from the 1930's when shopkeepers had to pay the tax man at the front door of their shops when receiving stock to sell, whereas the "crafty butcher" would have his MEAT delivered to the BACK DOOR.
"Check that guy, he's a crafty butcher, look at the way he walks."
"What do you mean?"
"Well he obviously takes meat round his back door."