Continuity mistake: As Marty watches George pedal away from the café, the amount of sunlight cast on the town square decreases greatly between shots. (00:41:00)
Continuity mistake: While Marty is talking with Jennifer in the square he is holding a tape case in his left hand which disappears in the next shots.
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."
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