C.S. Lewis’s dog was named Jacksie, and the animal inspired the nickname Jack that Lewis used for the rest of his life. The dog was killed by a car when Lewis was a child.
Lewis had been given the name Clive Staples at birth, but as a boy he announced that he would no longer answer to Clive. He chose the name Jacksie after his dog, eventually shortening or adapting it to Jack.
The nickname became deeply established among Lewis’s family and friends. It is why biographies and memoirs often refer to him simply as Jack, even though his published name remained C.S. Lewis.
Some accounts spell the dog’s name as Jacksie, while others discuss the nickname without emphasizing the spelling. The important biographical connection is consistent: the death of the childhood dog led Lewis to adopt Jack as his personal name. This detail is separate from his later use of the initials C.S.