Why do we say 'he let the cat out of the
bag'?
The origin of this phrase, meaning 'he gave
away a secret', dates back to the eighteenth century when it
referred to a market-day trick.
Piglets were often taken to market in a
small sack, or bag, to be sold.
The trickster would put a cat in a bag and
pretend that it was a pig.
If the buyer insisted on seeing it, he would
be told that it was too lively to risk opening up the bag, as the
animal might escape. If the cat struggled so much that the
trickster let the cat out of the bag, his secret was exposed. A
popular name for the bag itself was a 'poke', hence that other
expression 'never buy a pig in a poke'.