Did people anywhere from colonial times to ancient times, or any civilization with some sort of organized language with grammar rules, have any slang terms and/or platonic ways of speaking?
Hey. While there are some historical linguists here, this post might attract more responses in /r/linguistics.
BTW, by "platonic", do you mean "informal"? Languages do have different [registers](http://en.wikipedia.org/wiki/Register_(sociolinguistics)), which allow people to adapt their languages to different social settings. Hopefully some historical linguists can give you examples.