I think we have a generational thing to blame for the change of the word "thongs" - I grew up wearing them on my feet, now they're worn on the butt... apparently. Another word lost for general use. :-(
Idioms are fun - in fact, due to my writing, I have to end up writing my own idioms for my world of Terrenden.
One I've used recently is: "He is a dabbler hiding in the reeds!" to refer to someone who is afraid to act.
I also love the rhyming slang that you referred to. I learned a bit while I was in England. The younger generations will always come up with new euphemisms and idioms to encode their words away from their parents. The day this stops is the day the language dies...
Speaking of death - I think death (other than sex) creates the most colorful euphemisms/idioms... as demonstrated in Monty Python.
But bring up the word "idiom" and this is what comes instantly to mind: this scene with Steve Martin in The Pink Panther - "you, sir, are zee idiom!"
Ha ha! It's been ages since I'd seen the pink panther! It's amazing that you are creating your own idioms for your story universe! Is it difficult to do?
To begin with, yes. It requires a deeper understanding of your world than even most authors get to, I'm guessing. Especially when people do only one or two books or a tight hero's quest - rather than what I'm doing - a world which can handle (almost) everything I could ever want to write - so I have a lot of culture going on.
Another idiom developed in a book which hasn't really been even talked about on my updates yet - I think it got a mention once thus far - my original novel (which has to be reworked in the light of successive works) - this one was to "tread on a rotten pierf" (it may get reworked a little still) which referred to a fishing platform used by one of the Kri'enden tribes. But it means to get yourself into preventable trouble.
I also have a curse phrase "Flypp it!" which means pretty much what it sounds like - but... on Terrenden, its origin is in an early commercial from Flypp Industries who created the first popular versions of many types of technology devices including the flyppad and flyppcard and the humorously named flyppaper. Their ads said to "Flypp it!" This, of course, was humorous to the teens of the time, and evolved into a curse of its own.
Wow, it's great that you are able to develop such a background and lore! It would definitely help the reader get immersed as well!
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