LET THE CAT OUT OF THE BAG
To be truthful I don’t get the relevance of this phrase to its modern interpretation. Letting the cat out of the bag basically means revealing a secret. The ‘cat’ refers to the cat o’ nine tails, which was the ships whip, used to flog unruly sailors. Said ‘cat’ was kept in a cloth bag that was brought on deck for the floggings. So it was hardly a secret. Maybe it is all about sailors who were on the verge of becoming mutinous or the like and began plotting. Went on to talk to the wrong guy and were subsequently dobbed in. Hence the ‘cat’ was let out of the bag… That makes sense to me. Nasty though it is.