Re: @M. Poolman
To quote from http://www.wordorigins.org/index.php/site/let_the_cat_out_of_the_bag/
"It’s commonly asserted that let the cat out of the bag refers to the cat o’ nine-tails used on board ships as form of punishment. The whip would be kept in a special bag to protect it from the sea air and to let the cat out of the bag was to confess a crime worthy of flogging. A neat tale, except there is absolutely no evidence to connect the phrase with a nautical origin. "
AIUI there wasn't actually a "special cat" that was kept in a bag, rather someone who was due to be flogged would be given a short length of rope and they had to unravel it and actually make the device for their own punishment.
Naval ropes were made from three smaller ropes twisted together, each of which were made, in turn, from three thinner ropes, hence why the Navy Cat had nine tails.