What is wrong with Tom's left arm?
In To Kill a Mockingbird, Tom Robinson, a black man, has been accused of attacking and raping Mayella Ewell (a poor white woman and the daughter of the town drunk). Atticus is appointed to defend Tom at his trial and willingly does so despite the horrifyingly racist attitudes of the community. After showing that Mayella's attacker had to be left-handed based upon the location of her bodily injuries, Atticus asks Tom to rise so that Mayella can identify him. Tom does so, revealing a surprising deformity:
He looked oddly off balance, but it was not from the way he was standing. His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand...
Scout and Jem realize that Tom is crippled, and Reverend Sykes informs them that Tom was horribly injured in a childhood accident: his arm got caught in the cotton gin belonging to Mr. Dolphus Raymond which ripped his muscles loose from his bones. This is an important revelation for the court proceedings: it is clear that Tom could not possibly have attacked Mayella as Mayella had claimed due to his injured extremity.
In chapter 18, during Mayella Ewell's testimony in court, Tom Robinson stands up at Atticus's request so she can get a good look at him. Atticus has already established that someone with a good left arm, someone who is left-handed, beat up Mayella on her right side. The children get their first look at Tom when he stands up and they see that his left arm is almost a full foot shorter than his right. It is completely disabled and incapable of doing anything more than just hanging down at his side. Reverend Sykes leans over to Jem and clarifies the following:
"He got it caught in a cotton gin, caught it in Mr. Dolphus Raymond's cotton gin when he was a boy. . . like to bled to death. . . tore all the muscles loose from his bones" (186).
When Tom takes the stand in chapter 19, he can't even put his left hand on the Bible to swear his oath without using his right hand to help it. This proves that Tom Robinson did not beat up Mayella Ewell. He is physically unable to punch or do anything with his left arm.
Tom's left arm was seriously injured in a cotton gin accident when he was a boy. The muscles were all torn loose, so it is about a foot shorter than his strong right arm, and is essentially useless to him.