C.S. Lewis adopted the childhood nickname Jack after a dog named Jacksie died.
Lewis was christened Clive Staples Lewis, but around the age of four he began calling himself “Jacksie.” The name came from Jacksie, a family dog that was killed by a motorcar. Lewis reportedly refused to answer to any other name for a time.
The nickname was later shortened to Jack, and friends and family continued to use it throughout his life. “C.S. Lewis” remained his literary name, while “Jack” was the familiar name used by people who knew him personally, including his brother Warren and fellow members of the Inklings.
Readers sometimes assume that “Jack” was a pen name or a shortened form of Clive. It was neither: it was a chosen nickname rooted in a childhood memory. The detail helps explain why biographies and personal recollections usually call him Jack rather than Clive.