something acquired without inspection and which may therefore be worthless
Compare sell or buy a pup, let the cat out of the bag
Example They took the house for a month without knowing a thing about it, so it was a bit of a pig in a poke for them and I hope they won’t regret it.
(Note From agriculture: you would want to inspect a pig, etc. very carefully before buying it, and you’d be very unwilling to buy one wrapped up in a bag or ‘poke’ in case it only had three legs.)