C.S. Lewis’s first name was Clive, although his family later called him “Jack.” He was born Clive Staples Lewis in Belfast, Ireland, on 29 November 1898.
The nickname “Jack” arose after the death of his childhood dog, Jacksie. Lewis reportedly refused to answer to his given names for a time and adopted “Jacksie,” later shortened to “Jack.” The name remained his familiar personal name throughout his life.
Lewis used “C.S. Lewis” professionally as an author and scholar. The initials stood for Clive Staples, while “Lewis” was his surname. He did not use “Jack” as part of his published pen name, which helped distinguish his private identity from his literary reputation.