<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>
where did the saying the cat 's out of the bag come from
derivation of the phrase is not clear