Who let the cat out of the bag?

Letting the cat out of the bag is a colloquialism meaning to reveal facts previously hidden. One suggested origin of the phrase refers to the whip-like “cat o’nine tails”, an instrument of punishment once used on British Royal Navy vessels. The instrument was purportedly stored in a sack, and a sailor who revealed the transgressions of another would be “letting the cat out of the bag.”

See: https://en.wikipedia.org/wiki/Letting_the_cat_out_of_the_bag