C.S. Lewis’s childhood nickname, which he continued using as an adult, was Jack.
Lewis was christened Clive Staples Lewis, but as a boy he adopted the name “Jacksie” after the death of his dog, Jacksie. The nickname was later shortened to “Jack,” and friends and family continued to use it throughout his life. His initials, C.S., remained the form used on his published books.
The nickname is a common source of confusion because “Clive” appears in biographies and official records. “Jack” was not a pen name invented for publication; it was Lewis’s preferred personal name. Members of the Inklings, including J.R.R. Tolkien and Charles Williams, generally knew him as Jack rather than Clive.