C.S. Lewis’s childhood nickname was Jacksie.
The name came from a childhood incident involving a dog named Jacksie. After the dog was killed, the young Lewis reportedly insisted on being called Jacksie rather than Clive. The nickname was later shortened to Jack, the name used by friends and family for the rest of his life.
Lewis’s initials, C.S., came from his full name, Clive Staples Lewis. He generally preferred Jack in personal life, while C.S. Lewis became the public name attached to his fiction, apologetics, and scholarship.
Jacksie should not be confused with Warnie, the nickname of Lewis’s older brother Warren Hamilton Lewis. Warnie later became an important companion and biographer of his brother.