C.S. Lewis commonly used the nickname Jack throughout his adult life.
Lewis was born Clive Staples Lewis, but he became known to family and friends as Jack. The nickname originated after a childhood dog named Jacksie was killed; the young Lewis reportedly adopted Jacksie’s name and later shortened it to Jack. His brother Warren and many close friends continued using the nickname.
The name can be confusing because Lewis’s published initials were C.S., not J.S. or J.C. He generally signed his literary and academic work as C.S. Lewis, while “Jack” belonged mainly to his personal and social identity. Members of the informal literary group known as the Inklings also commonly addressed him by that name.
Using both names reflects the two sides of Lewis’s public life. “C.S. Lewis” identifies the author and scholar, whereas “Jack” evokes the friend, tutor, conversationalist, and private individual known to people around him.