C.S. Lewis’s lifelong nickname was Jack.
Lewis was born Clive Staples Lewis in Belfast on 29 November 1898. As a child, he chose the name Jack after the death of his dog, Jacksie, and eventually asked family members and friends to use it. The nickname became so established that “Jack” was the name used by people who knew him personally, even though his books appeared under the initials C.S. Lewis.
Lewis’s older brother, Warren, was commonly called Warnie. The brothers remained close and later shared interests in history, literature, and imaginative storytelling. Their childhood experiences helped shape Lewis’s later fiction, including his fascination with invented worlds and talking animals.
Readers sometimes assume that C.S. was Lewis’s preferred personal name. It was not: the initials were his professional author name, while Jack was the familiar name used in everyday life.