What does Scout mean when she describes Arthur Radley's communication as "body english"?
Scout observes Boo Radley's movements and describes his subtle actions as "body English." This is another term for body language.
As Scout and Jem walk home one evening, they are attacked by an angry Bob Ewell. Boo Radley comes to their rescue and fights Bob Ewell, but not before Jem's arm is broken in the scuffle. Boo Radley carries Jem back to the Finch house. He does not leave once he delivers Jem safely home. Instead, he hangs hesitantly against the wall in the room. Scout watches him, and she realizes he wants to touch Jem. Jem is laying in bed, resting. Scout encourages Boo to touch her sleeping brother:
His hand came down lightly on Jem's hair.
I was beginning to learn his body English.
His hand tightened on mine and he indicated that he wanted to leave. I led him to the front porch, where his uneasy steps halted. He was still holding my hand and he gave no sign of letting me go (Chapter 31).
It can be assumed that Boo Radley is hesitant and has awkward body language because of his lack of human interaction. He spends most of his time in his home, and he has only interacted with a few people over the years.