Arthur Radley exists in their environment as a ghost (or a "haint", to use regional colloquialism). His presence is felt, but never seen. He is the source of "ghost stories," such as stabbing his father in the leg with the scissors. He wanders just outside of the peripheral vision until the end. Despite Atticus's insistence that they call him "Mr. Radley," and give him the respect an adult in the community deserves, he remains "Boo" to the end.
Boo is meant to be a scary, mysterious character in the children's lives. Therefore he has been named by the people around him who are unsure of just how crazy or scary Arthur Radley is. Boo's name represents the town's fear of all things unknown. The Ewells' desperation in accusing Tom Robinson of rape can be interpreted as another aspect of the fear assigned to the significance of Boo's name. You'll notice whenever Jem or Scout or Dill refer to him throughout most of the book, his name is said with force and they are frightened of the consequences of actually encountering him. However, the entire book turns when Scout actually meets Boo. Her calling his name is not done with fear and trembling as the name might suggest, but is done with quiet concern for the poor man behind the door. Atticus introduces Scout to him as Mr. Arthur Radley. And when she takes him out onto the porch to sit, she address him as Mr. Arthur. The fear the name held and the mysterious person represented by the name have disappeared. In its place is Mr. Arthur who saved her and her brother's life that fall night.
This nickname is never really explained to the reader, though one could imagine it is a result of his reputation. The summary found here on enotes states that "Boo is rumored to wander the neighborhood at night", much like a ghost which could explain the nickname. Furthermore, the dark acts which he was rumored to have committed, such as stabbing his father in the leg with scissors, eating animals, and causing all kinds of mischief definitely lends alot to the town's need for the nickname, indicating a fear of Arthur Radley and representing the town's fear of the unknown.
In "To kill a mockingbird" book, the children calls Arthur Radley Boo because they have never seen him outside the house and only heard mysterious story about him.
A common choice of wording for children back in the days. When one person scares another person, the typical thing for them shout is 'BOO!' Back then, "boo" was commonly referred to as the noise a ghost makes. Subsequently, since Arthur does play the role of a mysterious, locked away, child. They gave him the nickname "Boo".
(A silly answer of mine is, try thinking about the Mario Bros. series. Remember that little ghost character named "Boo"?)
His real name is Arthur Radley. "Boo" is the name the children give him, because they think of him as a sort of phantom.
no boo radley is simply the horrific name the neighborhood gave him because they thought he was a bad person.
because he's supposed to be paying the role of a mysterous person and the name boo sounds scary therefore, he's the monster in the childrens lifes.