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 An in-joke, also known as an inside joke or a private joke, is a joke whose humour is understandable only to members of an ingroup; that is, people who are ''in'' a particular social group, occupation, or other community of shared interest. It i ...

) or the revelation of a

plot twist A plot twist is a literary technique that introduces a radical change in the direction or expected outcome of the plot in a work of fiction. When it happens near the end of a story, it is known as a twist or surprise ending. It may change the aud ...

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 The cat o' nine tails, commonly shortened to the cat, is a type of multi-tailed whip or flail that originated as an implement for severe physical punishment, notably in the Royal Navy and British Army, and as a judicial punishment in Britain ...

", 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".Let the Cat out of the Bag
at Snopes.com; by Barbara Mikkelsen; published 16 July 2012; retrieved 31 May 2013 Another suggested derivation is from the "

pig in a poke A ''pig in a poke'' is a thing that is bought without first being inspected, and thus of unknown authenticity or quality. The idiom is attested in 1555: I wyll neuer bye the pyg in the poke Thers many a foule pyg in a feyre cloke A "poke" is a b ...

" scam, where a customer buying a

suckling pig A suckling pig is a piglet fed on its mother's milk (i.e., a piglet which is still a " suckling"). In culinary contexts, a suckling pig is slaughtered between the ages of two and six weeks. It is traditionally cooked whole, often roasted, in ...

in a sack would actually be sold a (less valuable) cat, and would not realise the deception until the bag was opened.

Johannes Agricola Johann or Johannes Agricola (originally Schneider, then Schnitter; 20 April 1494 – 22 September 1566)John Julian: Dictionary of Hymnology, Second Edition, page 19. London: John Murray, 1907. was a German Protestant Reformer during the Protestan ...

made reference to the expression "let the cat out of the bag" in a letter to

Martin Luther Martin Luther (; ; 10 November 1483 – 18 February 1546) was a German priest, theologian, author, hymnwriter, and professor, and Order of Saint Augustine, Augustinian friar. He is the seminal figure of the Reformation, Protestant Refo ...

on 4 May 1530 as referenced in Lyndal Roper's 2016 biography about Martin Luther. Both of these suggestions are rejected by Snopes.com, who find no evidence of it originating in naval slang, nor of whips being stored in sacks, and consider it "nigh on impossible to mistake a cat for a pig".

References

{{Use dmy dates, date=May 2019 English phrases Metaphors referring to cats