“Knee high to a grasshopper” (short)
“Beyond the pale” (outside social norms, I think. Maybe just strange)
“Dance maven”
“I’ll just do that in my copious free time…” (sarcastically, because you are too busy)
“Copacetic” (it’s all good)
“Heavy” (meaning important, grave)
I learned all of these thirty years ago from a man in his fifties. He was full of interesting expressions.
The strange thing here is that Marty McFly is using the slang “heavy” and Doc Brown doesn’t understand it. Yet the use of heavy to mean “profound, serious” started in Jazz in 1937. So Doc would have grown up with the word, and by the 80’s I wonder if it was really all that popular anymore. McFly is using 50’s slang in the '50s but the guy from the '50s doesn’t get the slang. Maybe because he’s not hip.