@The Psyentist 542928 wrote:

Well it is used to mean you don’t want to do something and a broken leg is a pretty justifiable excuse for not doing a lot of things. A slapped wrist isn’t likely to teach you much or prevent you from doing something though is it. I feel like I’ve let myself down by responding sensibly there 🙁

That may be what it means in your crazy northern world mate, but down here it has always meant ‘Good luck’. I think it’s an old theatrical saying/superstition, kinda similar to the ‘Don’t say “Macbeth”‘ bollocks.