Scout, or Jean Louise Finch, is the character who represents as a girl the author herself, Harper Lee. Having lost her mother at an early age, and living, thus, in a male household, perhaps the feminine name of Jean Louise was perceived as too prissy for such a hoyden as the sister of Jem. Possessive of a curious nature and fiercely loyal to her father, perhaps he gave her the moniker of Scout.
It is interesting that Aunt Alexandra is the presence who issues the ordinance that Scout must wear dresses, act ladylike, and re-assume her birth name. For, she is the member of the Finch family who holds stock in the family background and its prestige. In Chapter 13, Aunt Alexandra advances upon Scout,
"We decided that it would be best for you to have some feminine influence. It won't be many years, Jean Louise, before you become interested in clothes and boys--"
After this, Jean Louise wears the habiliments of a dainty girl when she attends the Missionary Tea that Aunt Alexandra hosts at the Finches home.
Scout's full name is Jean Louise Finch.