Final answer:
Tom Robinson's left hand was handicapped from an accident with a cotton gin when he was a boy, which plays a crucial part in proving his innocence in 'To Kill a Mockingbird'.
Step-by-step explanation:
Tom Robinson's left hand became handicapped due to a cotton gin accident when he was a young boy. The machine injured his arm so severely that it left him without the use of his left hand. In the book 'To Kill a Mockingbird', this injury is significant as it relates to the critical plot point concerning his physical inability to commit the crime he is accused of. The novel demonstrates Tom Robinson's innocence through the disability of his left hand, highlighting the unjust nature of his conviction largely based on racial prejudice.