I first heard the word copacetic in college in the 1960s. The guys in the dormitory, for a time, loved to play with the word. It was fun to say, and not everyone knew what it meant. Once everyone learned what it meant (satisfactory, OK, excellent, acceptable or fine), the word seemed to die out for lack of use. I have rarely heard or seen it used since.
Curiously, the book Flappers 2 Rappers: American Youth Slang dates the word from the 1920s, which would have been long after Robinson, born in 1878, was a boy. The American Heritage Dictionary does not label copacetic slang at all. The literary quotation it cites comes from novelist John O'Hara, who wrote, "You had to be a good judge of what a man was like, and the English was copacetic."
I wrote a few days ago about how English words with Latin or Greek origins carry more clout than simple Anglo-Saxon words. Much of copacetic's appeal is that it both sounds and looks like it must have come from Latin or Greek. Yet as far as anyone knows, it is just a made-up word. If Bill Robinson didn't invent it out of thin air, somebody else did.