C.S. Lewis’s childhood nickname was Jack.
Lewis was named Clive Staples Lewis, but he disliked the name Clive from an early age. After the family dog, Jacksie, was killed in an accident, the young Lewis reportedly insisted on being called Jacksie. The nickname was later shortened to Jack, and it remained the name used by friends and family for the rest of his life.
This explains why biographies and reminiscences often refer to him as Jack Lewis rather than Clive Lewis. “C.S. Lewis” was his published name, while “Jack” was the personal name associated with his everyday life. “Staples” was his middle name, not his familiar nickname.
The nickname also appears in the names of Lewis societies and memorial institutions. It is especially useful for distinguishing biographical references to Lewis from references to his professional and literary identity, which nearly always use his initials.