C.S. Lewis’s childhood nickname was Jack.
Lewis was born Clive Staples Lewis in Belfast in 1898. When he was about four years old, his dog Jacksie was killed by a car. Lewis then announced that he would be called “Jacksie,” later shortened to “Jack.” The name stayed with him throughout his life, and friends, colleagues, and family generally used it rather than Clive.
The nickname is sometimes confused with “Jacksie,” which was the earlier form, or with “C.S.,” the initials he used as an author. Lewis’s brother Warren was often called Warnie, another family nickname that remained in use.
Lewis published fiction and scholarship under C.S. Lewis, but letters and biographies frequently identify him simply as Jack. The nickname therefore refers to the man behind the well-known authorial initials, not to a separate pen name.