C.S. Lewis’s childhood nickname was Jack.
Lewis was born Clive Staples Lewis, but he adopted the name Jack after a childhood incident involving a dog named Jacksie. According to accounts of his early life, he began calling himself Jacksie and later shortened the name to Jack. Friends and family continued to use Jack throughout his life.
The nickname explains why letters, memoirs, and biographies often refer to him as Jack rather than Clive. His brother Warren was also known as Warnie, creating a familiar private vocabulary within the Lewis family. Readers sometimes assume that “Jack” was a pen name, but it was primarily his personal name among people who knew him.
His public literary identity was different. He normally published under the initials C. S. Lewis, a form that became internationally recognizable through works including The Problem of Pain, The Screwtape Letters, and the Narnia books. The contrast between “Jack” in personal life and “C. S. Lewis” in print remains one of the best-known details about the author.