<Li> revealing a conspiracy (friendly or not) to its target </Li> <Li> in a movie or play, the revelation of a plot twist </Li> <Li> letting an outsider into an inner circle of knowledge (e.g., explaining an in - joke) </Li> <P> 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 Martín Luther . </P>

Who let the cat out of the bag