Something that is offered in a manner that conceals its true nature or value.
It is also phrased as 'to buy a pig in a poke', meaning to buy something when you don't know what it is, or haven't examined it properly. Buying a used car without test driving it, for example.
This comes from the English farmers of yore, who carried pigs to market in a small bag, AKA, a poke. A dishonest pig vendor might claim that he could not open the bag for fear the pig would get loose, in order to hide that fact that the pig was a runt, or worse, not a pig at all, but a cat. (Hence, the phrase Let the cat out of the bag* also entered our language).
* Sverre reports that in Norwegian there is a saying, "to buy the cat in the bag" -- giving us a nice connection between the two sayings.