As an idiom, 'a cat in the bag' means nothing. The only expression we have involving cats and bags is the phrase 'let the cat out of the bag', which means to reveal something which should have been kept secret. For example 'We were trying to keep Grandma's surprise birthday party a secret, but unfortunately my little brother let the cat of the bag and told her about it.'

A secret, I guess. There is the expression: "to let the cat out of the bag", which means to reveal a secret... so a cat in the bag must be a secret :). https://www.facebook.com/idioms.through.pictures/photos/a.656600267818041.1073741827.656595417818526/723128724498528/?type=3&theater

It means that you got something and you do not know exactly what you've got (good thing or bad)

A secret, I guess. There is the expression: "to let the cat out of the bag", which means to reveal a secret... so a cat in the bag must be a secret :). https://www.facebook.com/idioms.through.pictures/photos/a.656600267818041.1073741827.656595417818526/723128724498528/?type=3&theater