At first glance, this might seem like a stange expression. After all, what was the cat doing inside the bag in the first place in order to be let out of it?? And why should this be regarded as a way to reveal a secret?
The expression dates back to the 1700s. The idea is that an unscrupulous trader might try to sell a sack which was supposed to have a pig in it – a useful animal, which could be used for meat and leather, while in reality a cat had been put in the sack. Obviously, if the cat was let out of the bag, then you would realise that you were being had – the secret would be revealed too early. One can only assume that the cats put in these bags were not the noisy type, else the secret would be revealed by a loud meow, without having to let the animal out at all!
There has been some speculation that the expression is related to the “cat-o-nine-tails”, a kind of whip used to punish sailors. However, there does not seem to be any basis for this claim. Taking a known whip out of a bag seems to bear no relation to the idea of revealing a secret, and in fact the only thing our idiom has in common with this is the word “cat”, hardly an uncommon animal, and one that we see used in a number of other expressions too.
Interestingly, it wasn’t just merchants in Britain who were dishonest and tried this trick – exactly the same expression exists in German; die Katze aus dem Sack lassen, and in North Welsh at least, it is again the same as in English gollwng y gath o’r cwd.
In Poland people are still taking things out of bags; wyszło szydło z worka, which means “an awl came out of the bag”, an awl being a kind of tool used to make holes in leather.
The Spanish have a number of expressions; soltar la lengua, which translates as “release the tongue” is the most literal when it comes to revealing secrets. Then there is abrir la bocota, “open the big gob”, which of course we might say in English too, and my personal favourite, destapar la olla, which means “uncover the saucepan”. To me at least, this brings a rather lovely image to mind of a pot bubbling over with secrets waiting to be revealed!
The French expression, itself dating only to the 19th century, is quite explosive! Vendre la mèche – “to sell out (betray) the fuse”, in reference to a way of discovering where mines were.
This brings me on to Slovenian, where we find a familiar expression. Daj karte na mizo, which literally means “lay your cards on the table”, in order to reveal a secret. However, there is another expression in Slovene that I rather like. The usage is slightly different, but it is still on the general theme of revealing secrets. When someone has something to say and doesn’t know where to start, people may prompt them with kaj imaš za bregom, which translates as “what do you have behind the hill”. This is a lovely note to finish on and leave you wondering what I have waiting behind the next linguistic hill!