You may remember that idioms are groups of words that can be confusing because the meaning of the whole group of words taken together has little to do with the meaning of the words taken one by one. To understand them you have to know the hidden meaning.
I’ll let one cat out of the bag-many idioms reference animals.
This idiom means: to give away a secret
It’s believed to have it’s origins centuries ago when untrustworthy merchants would put a cat in a bag that was suppose to contain a piglet.
Here are a couple more animal idioms.
Don’t tell your friend you forgot her birthday. Better to let sleeping dogs lie.
This idiom means: don’t stir up trouble if you don’t have to
This idiom dates back to the 1200’s. If you came upon sleeping dogs it might be best to let them sleep since you don’t know what they might do if you woke them.
I’m going to wait to walk to the car because it’s raining cats and dogs.
This idiom means: to rain heavily; a downpour
One theory about this idiom is that during heavy rains in 17th century England, cats and dogs would drown and their bodies would be found on the street as if they’d fallen from the sky with the rain.