Origin and meaning of "Let the Cat out of the Bag"

One of those "where did that come from" expressions. The meaning is straight forward. To give a secret away.
Would appear as though in days of past one would trot off and buy a pig from your local butcher at the market. The pig would be placed in a bag for convenience. (The bag was called a "poke"). A dishonest butcher would replace the pig with a big cat.
Now, if you opened the bag later, to find out that you had been taken for a ride, the cat was "let out of the bag", the Butchers secret was exposed.
Back to the poke. If you buy a pig in a poke you have bought something worthless. From the same origin.
A number of sources say this is not true, the cat referred to is the cat 'o nine tails, used to flog sailors in the Royal Navy. The whip was kept in a bag, and when taken out, it meant that the flogging was a certainty.
There you have it.....