C.S. Lewis adopted the childhood nickname Jacksie after his dog, who was called Jacksie, died in 1908.
Lewis was known as Clive Staples Lewis, but he disliked the name Clive from an early age. After the death of his dog, he announced that he would be called Jacksie instead. The name was later shortened in everyday use to Jack, which became the name used by friends and colleagues throughout his adult life.
The story is a small but revealing detail in Lewis’s biography. His early attachment to animals, invented worlds, and private names foreshadowed interests that later appeared in his fiction. His brother Warren also became known as Warnie, another familiar family form.
Jacksie is sometimes mistakenly written as Jackie or treated as Lewis’s formal name. It was neither: Clive Staples Lewis remained his legal name, while Jacksie and later Jack were personal forms of address. “Jack” is therefore the canonical adult nickname associated with Lewis.