Meaning of 'Cat out of the bag'

[Padmanabhan.J]

Not long ago in England, a city dweller had to be careful of buying a pig from a villager who would keep it in a sack. Very often, cunning villagers’ intent on duping the customer substituted the pigs with cats. And when the city dweller opened the bag, he would literally let the cat out of the bag, revealing the crafty farmer's secret.