C.S. Lewis used the nickname Jack among friends and colleagues.
His full name was Clive Staples Lewis, but he adopted Jack as his preferred personal name during his youth. The nickname became associated with him throughout his adult life, including his years at Oxford and Cambridge. Close friends, family members, and members of the literary discussion group known as the Inklings generally called him Jack rather than Clive.
Lewis reportedly chose the name after a childhood dog named Jacksie was killed. He initially used forms such as Jacksie before the shorter Jack became established. The story is often included in biographies because it shows how strongly Lewis connected personal identity with names and private memory.
C.S. Lewis remained the name used on his books and in public discussion. “Clive” is his legal first name, while “Staples” is his middle name, but neither was the familiar form preferred by his circle. Warren, or Warnie, was instead the nickname of his older brother.