Always examine your purchase before you pay for it. A poke is a bag or sack. In olden days a crafty trader would tie a can in a sack and sell it to some trusting simpleton as a piglet, much more valuable than a mere cat. If the would-be buyer was wise enough to insist on seeing the piglet before parting with his money, the trader’s dishonesty would be revealed. Hence the proverbial expression ‘to let the cat out of the bag’, which means, in a figurative sense, to give away a secret.