The dog whose death inspired C.S. Lewis to call himself Jack was named Jacksie.
Lewis adopted the name after his childhood dog Jacksie was killed by a car in 1908. Lewis was about nine years old, and he reportedly announced that he would no longer answer to his given name, Clive. The nickname was later shortened to “Jack,” which became the name used by family, friends, and colleagues for the rest of his life.
His formal name was Clive Staples Lewis, and “C.S. Lewis” became his professional authorial name. The contrast between the public initials and the private nickname can make accounts of his life seem to refer to different people, but Jack was simply the familiar name he chose.
The episode is often mentioned in biographies because it illustrates Lewis's intense emotional attachment to animals and his early capacity for imaginative self-definition. It also predates his later academic and literary identity by many years. Lewis's brother Warren, usually called Warnie, continued to use Jack as his brother's everyday name.