C.S. Lewis used the nickname Jack among friends.
Although Lewis was born Clive Staples Lewis, he became known personally as “Jack.” The nickname originated after the death of a childhood dog named Jacksie, when the young Lewis reportedly began insisting that he himself be called Jacksie. The form eventually shortened to Jack.
Family members, friends, and colleagues commonly used Jack rather than Clive. His initials, C.S., remained the public signature on his books, while Jack reflected the more personal identity known to people close to him.
The nickname is sometimes mistaken for a pen name, but it was not. “Clive Hamilton,” for example, was a publishing pseudonym used for The Pilgrim’s Regress. “Jack” was an informal personal name that continued throughout Lewis’s adult life.