I always thought the "let the cat out of the bag" referred to the cat o'nine tails the ship's captains used to whip unruly crewmen. Don't let the cat out of the bag referred to not letting on to the plan or the plot and thus unleash the whip.

When I was learning Russian, we were taught that a large portion of Russian is idiomatic and cannot be translated literally. For example, дождь идёт translates literally as "rain takes a walk," but really means "It is raining." This makes learning Russian even that much more difficult than English, not to mention the complex grammar in Russian with multiple declensions and cases for nouns and adjectives (even more than in Latin) and multiple ways of conjugating verbs, not counting all the irregular verbs. We were also taught that most Russians do not speak proper Russian - mainly because it is too difficult. English is much simpler and easier, and still native English speakers often do not speak proper English. It's too bad that Latin is not taught as much in schools as it used to be.

It's also in A Christmas Carol. I had forgotten all about that one and I have several books illustrated by Arthur Rackham. https://victorianweb.org/art/illustration/rackham/7.html

I wonder if that's related to dancing a jig...if that was considered a 'trick' move or not.