C.S. Lewis used the nickname Jack among friends and family.
Lewis was born Clive Staples Lewis in Belfast in 1898. As a child, he called himself “Jacksie” after the death of his dog, which had been named Jacksie. The childhood form was later shortened to “Jack.”
The nickname became the normal way that close friends, relatives, and colleagues addressed him. His initials, C.S., remained his public author name, while Jack identified the private person behind the books.
Readers sometimes assume that Jack was a pen name used for fiction, but it was principally Lewis’s personal nickname. “Clive Staples Lewis” remained his legal and formal name, and “C.S. Lewis” was the name used on his published works.