C.S. Lewis used the name Jacksie for his childhood dog before adopting the nickname Jack. Lewis’s birth name was Clive Staples Lewis, but at about four years old he chose to be called Jacksie after the family dog was killed by a car. The name was later shortened to Jack.
The nickname became the name by which Lewis was known to friends, colleagues, and readers throughout his adult life. It was not his legal name, and formal biographies generally identify him as Clive Staples Lewis or C. S. Lewis. Warren Lewis, his older brother, was among the people who used Jack in family and personal contexts.
This small biographical detail is often mentioned because it shows how Lewis’s public identity developed from a childhood experience. The name also helps explain why many letters and memoirs refer to him simply as Jack. His later writing made “C. S. Lewis” the familiar literary form, especially on book covers.