Final answer:
Atticus's nickname when he was a boy in To Kill a Mockingbird was One-Shot Finch because he was an exceptional marksman.
Step-by-step explanation:
In To Kill a Mockingbird, Atticus's nickname when he was a boy was One-Shot Finch. He got this nickname because he was an exceptional marksman and had the ability to shoot accurately with only one shot. Atticus's aim was so precise that he rarely needed a second shot when hunting or shooting targets.