Letting the cat out of the bag

Letting the cat out of the bag (also ...box) is a colloquialism meaning 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 is also known as to reveal a secret carelessly or by mistake.

Etymology
The derivation of the phrase is not clear. One suggestion is that the phrase refers to the whip-like "cat o'nine tails", an instrument of punishment once used on Royal Navy vessels. The instrument was purportedly stored in a red sack, and a sailor who revealed the transgressions of another would be "letting the cat out of the bag". Another suggested derivation is from the "pig in a poke" scam, where a customer buying a suckling pig in a sack would actually be sold a (less valuable) cat, and would not realise the deception until the bag was opened. Johannes Agricola made reference to the expression "let the cat out of the bag" in a letter to Martin Luther on 4 May 1530 as referenced in Lyndal Roper's 2016 biography about Martin Luther, "Martin Luther: Renegade and Prophet".