C.S. Lewis’s childhood nickname was Jacksie. He adopted it after a childhood dog named Jacksie was killed in a road accident when Lewis was four years old.
Lewis’s birth name was Clive Staples Lewis, but after the dog’s death he reportedly rejected the name Clive and insisted on being called Jacksie. The nickname later developed into Jack, the form used by friends and colleagues for much of his life. Warren Lewis, his brother, also helped preserve details of this family history in his memoirs.
The name Jack is therefore not simply a shortened form of Clive or Staples. It originated in a private childhood episode and remained part of Lewis’s personal identity. Many biographies refer to him as Jack Lewis even though his books were published under the initials C.S. Lewis. Readers can also encounter the name Jacksie spelled in slightly different ways in secondary accounts, but Jacksie is the form commonly associated with the story of the dog.