Pig-in-a-poke refers to a scam originating in the late middle ages, when meat was scarce, but apparently rats and cats were not.
The scheme entails a sale of a "suckling pig", in a "poke" (bag), the bag obviously containing a live healthy little pig. The wriggling bag actually contains a cat, not partcularly prized as a source of meat, and at any rate, quite unlikely to grow to be a large hog.
The bag which ostensibly conains a baby pig is sold to the victim, without opening it.
A common colloquial expression in the English language, meaning "to allow oneself to be easily conned", or "one gets what they pay for" is to "buy a pig in a poke". Similar expressions exist in other languages, e.g. in German "die Katze im Sack kaufen" (lit.: to buy the cat in a bag).
This is also the origin of the expressions: "Let the cat out of the bag" meaning 'to reveal that which is secret', and "left holding the bag" meaning 'to find oneself with nothing for their efforts', as the cat is quite likely to flee when the bag is opened.
See also: Fraud, Confidence trick, Livestock
