Other mistake: The car that almost runs Marty over has the vehicle registration plate of 6S 48405. Later on, when Marty is hit by Sam, Sam's car has the same plate. (00:35:49 - 00:42:02)
Other mistake: When the Libyans shoot at Doc Brown and the bullets hit his van they leave no bullet holes, just impact sparks. (00:29:10)
Other mistake: When young George and Marty are in the backyard talking about what will happen at the dance, young George hangs up clothes to let them dry, but if you look closely the clothes are actually already dry.
Answer: It's a way of saying "scram" or "get lost." But Biff is so dim, he doesn't realize he's saying it wrong; the expression is "make like a tree and leaf", with the joke being that "leaf" is meant to sound like "leave."
Cubs Fan ★