where did the term let the cat out of the bag come from
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 the 4th May 1530 as referenced in Lyndal Roper's 2016 biography about Martín Luther.