Letting the cat out of the bag
Look up let the cat out of the bag in Wiktionary, the free dictionary.
Letting the cat out of the bag (also ...box) is a colloquialism that means to reveal facts previously hidden. It could refer to revealing a conspiracy (friendly or not) to its target, letting an outsider into an inner circle of knowledge (e.g., explaining an in-joke), or the revelation of a plot twist in a movie or play. It also means to reveal a secret carelessly or by mistake.