C.S. Lewis used the nickname Jacksie after a childhood dog named Jacksie was killed. Lewis was about four years old when he declared that he would no longer answer to “Clive” and would instead be called Jacksie.
The nickname was later shortened to “Jack,” the name used by family and close friends for much of his life. This explains why biographies and personal recollections often refer to Lewis as Jack rather than Clive.
The episode belongs to Lewis’s early childhood in Belfast and is frequently mentioned because it shows how strongly he identified with stories, names, and imaginative worlds from an early age.
“Jack” was not his legal name, and “Jacksie” was not a literary pseudonym. His published name remained C.S. Lewis, while the nickname was part of his private and social identity.