C.S. Lewis adopted his childhood nickname after his dog Jacksie.
In 1908, when Lewis was nine years old, his small dog Jacksie was killed by a motorcar. Lewis then refused to answer to his given name and began calling himself Jacksie. The name was later shortened to Jack, which became the familiar form used by his family, friends, and colleagues throughout his life.
This episode helps explain why biographies of Clive Staples Lewis frequently refer to him as Jack. The nickname was not a literary pen name and was not derived from the initials “C.S.” Instead, it came from a personal event in his childhood.
Lewis’s brother Warren was also known by a nickname, Warnie. Because “Jack” became so closely associated with Lewis, readers sometimes assume it was his birth name. His formal name remained Clive Staples Lewis, while Jack was the chosen everyday name that followed the loss of Jacksie.