C.S. Lewis’s childhood nickname, taken from his dog’s name, was Jack.
Lewis was born Clive Staples Lewis, but as a child he adopted the name “Jacksie” after a beloved dog named Jacksie was killed by a car. He eventually shortened the nickname to “Jack,” and friends and family generally used it for the rest of his life.
The name explains why biographies and letters often refer to him as Jack rather than Clive. His brother Warren was also known as Warnie, another family nickname that appears frequently in accounts of their lives together.
Lewis’s initials remained part of his public author name. “C.S. Lewis” appeared on his books, while “Jack” belonged more to his private and personal identity. Clive Staples was therefore his formal name, but Jack was the name by which he was widely known among people close to him.