I was reading a book a couple of months ago about the Royal Air Force and was surprised to find that a lot of the language the pilots used in WW II was part of my childhood vocabulary almost 30 years after the war ended. Those guys sure made an impression. Up until I read this book I had no idea that “jammy”, which we used for undeserved luck, was RAF slang. Is there such a thing as deserved luck? But I digress.
How different my generation was from the youngsters today who want to speak like urban American black “gangstas”. At least the guys we were taking a lead from, albeit without realising it,  helped save the world for democracy, or at least paid a key role in holding back the Nazi hordes when Britain stood alone – if you don't count India, Canada, Australia, New Zealand, South Africa, the West Indies, and other odds and sods from the Commonwealth/Empire.
Or so I thought. More recently I was reading a book about the first penal colonies in Australia. I found that many of the criminal exiles spoke a language unintelligible to non-criminal outsiders. It was called “flash” or “cant”. And lo and behold the word for a juvenile who grabbed stolen property from a thief and darted away through the crowd with the evidence was “kiddy”. I remember my grandpa always talked about “the kiddies” and I'm sure he didn't mean child criminals who should have been behind bars. I refer to small children as “kids” all the time. So, I use 18th Century criminal slang all the time. I wonder what “rap” lingo will be used in 200 years time. I'm sure some of it will.
In a third book, yes I do read a lot, I came across the surprise origin of a word a lot of my school mates used for 'crazy”, usually “fighting crazy”. The word was “raj”. It turns out the word in Romany in origin. I would never have guessed. There was time when a gypsy was as welcome in the average home as a modern-day rapper determined to prove he's still “real”. Or whatever the term is.