C.S. Lewis commonly used the name Jack among family and friends.
The nickname arose from a childhood incident involving a dog named Jacksie. After the dog was killed, the young Lewis reportedly insisted on being called “Jacksie,” a name that later shortened to “Jack.”
Although his legal name was Clive Staples Lewis, “Jack” became the name used by people who knew him personally. His brother Warren was commonly called “Warnie,” creating a familiar pair of nicknames in accounts of their household.
Readers sometimes confuse Jack with a pen name. It was not a separate author identity: it was Lewis’s everyday personal name. His published books appeared under C.S. Lewis, while friends and colleagues often addressed him as Jack.