"Letting the cat out of the bag" has come to mean giving away a secret; to reveal facts previously hidden.  Never mind how hard it would be to actually put a cat in a bag... any bag... any cat.  The bigger question is why somebody would want to?  And the answer to that is to fool gullible people, in particular, anybody who is gullible enough to buy a pig without first seeing it, say, if it were a pig in a poke.  A "poke" is an older, more rustic name for a bag.  Of course, there needs to be something in that bag other than a pig to pull off the scam, and what better animal than a cat?  The rube who is naïve enough to buy that pig sight-unseen, and who can't tell the difference between a pig's squealing and a cat's caterwauling, won't realize he's been had, of course, until he lets the cat out of the bag.

(Katharine M. Rogers from Cat, Reaktion Books, 2006)