Where'd this expression, and the rules surrounding it, come from?
No one knows. It's just been around that long. It's not even correct: The word "in" does not mean "in" when speaking Italian. The word "nella" translates to "in." Is "in" just a mistranslation? Someone wrote it down wrong? Is it really n'boca with the n' serving as a contraction of "nella?"
When I first heard this expression so many years ago, I thought it was about the legend of the founding of Rome where Romulus and Remus were saved by a mother wolf and suckled; Therefore to be in the mouth of the wolf would mean to give over your troubles to a benefactor. Is it a message to relax because you are not in control?
Some have pointed out that the last word is lup-O not lup-A, so therefore it's not about Romulus and Remus, it's about hunters who would kill wolves which plagued towns in Italy. It would also account for the violent response. My problem with this idea is that 1- you don't want to be in the wolf's mouth because you'd be injured or killed so you're a crappy hunter in that case. 2- Just because it's now written as lup-O doesn't mean that's how it started, just how it was written. Again, Italian isn't a precision language! 3- the "crepi" response could have been added at any time.
The congruence with "break a leg" is tenuous too. "Break a leg" isn't a wish of bad luck, it's a wish that you get to bow or curtsey (hence the leg breaking part) to applause after a performance.
And just to show you that I didn't make this all up tonight, here's an excerpt from another source, and a fun, brief article to read.
In Italy, it is good luck to touch iron. Knocking on wood will do nothing for you. It is also good luck to touch the hump of a hunchback or stroke the nose of a wild boar. Fortune will also smile kindly upon you if you jump when you see a priest or play 98 on the lottery wheel after you dream of a dead relative. I am told that rain on your wedding day is really good too.
It also may be useful to know that in Italy, it is considered extremely bad luck to wish someone good luck in the literal way by saying buona fortuna. The lucky way to wish good fortune is to say in bocca al lupo, which can be translated as “into the wolf’s mouth.” Similar to the English expression “break a leg,” the “in bocca al lupo” metaphor compares any challenging scenario to being caught between the hungry jaws of a wild beast whose aim is to swallow both the misfortunate and the careless. The “wolf” may be a teacher who wants to fail you, a critical audience before a presentation at work, or simply symbolize having to overcome any excruciatingly difficult enterprise.
There are other similar sayings in Italy, more vulgar versions of our Wolf wishes, which I think sum things up nicely for tonight:
"In groppa al riccio!" ("On an hedgehog's back!")
To which one replies:
"Con le mutande di ghisa!" ("Wearing underpants made of cast iron!")
I like this one. Especially since touching metal is lucky, you accomplish two things at once!
The last one for tonight:
"In culo alla balena!" (In the ass of the whale!)
Spero che non caghi. (I hope it doesn't shit.)
This phrase is way less mysterious. I don't know what that guy is doing in a whale's ass, but for his sake I hope it doesn't shit either!
While searching the internet tonight I think I found the answer!