A pig in a poke
An offer or deal that is foolishly accepted without being examined first.
'Don't buy a pig in a poke' might seem odd and archaic language. It's true that the phrase is very old, but actually it can be taken quite literally and remains good advice.
The advice being given is 'don't buy a pig until you have seen it'. This is enshrined in British commercial law as 'caveat emptor' - Latin for 'let the buyer beware'. This remains the guiding principle of commerce in many countries and, in essence, supports the view that if you buy something you take responsibility to make sure it is what you intended to buy.
A poke is a sack or bag. It has a French origin as 'poque' and, like several other French words, its diminutive is formed by adding 'ette' or 'et' - hence 'pocket' began life with the meaning 'small bag'. Poke is still in use in several English-speaking countries, notably Scotland and the USA, and describes just the sort of bag that would be useful for carrying a piglet to market.
A pig that's in a poke might turn out to be no pig at all. If a merchant tried to cheat by substituting a lower value animal, the trick could be uncovered by letting the cat out of the bag. Many other European languages have a version of this phrase - most of them translating into English as a warning not to 'buy a cat in a bag'. The advice has stood the test of time and people have been repeating it in one form or the other for getting on for five hundred years, maybe longer.
Fraser's Magazine, 1858, reprinted a piece from Richard Hill's (or Hilles') Common-place Book, 1530, which gave this advice to market traders:
"When ye proffer the pigge open the poke."
John Heywood included something nearer to our modern-day version of the phrase in Proverbes and Epigrammes, 1555-60:
I will neuer bye the pyg in the poke :
Thers many a foule pyg in a feyre cloke.
See also: 'let the cat out of the bag'.