The nickname “Jack” referred to C.S. Lewis’s first name, Clive.
Lewis was born Clive Staples Lewis, but he became known to family and friends as Jack. The nickname originated in childhood after the death of his dog Jacksie; Lewis reportedly adopted “Jacksie” and then “Jack” as names for himself.
The name “Jack” remained part of his personal identity throughout adult life, even while his books appeared under the initials C.S. Lewis. Friends, colleagues, and members of his Oxford circle commonly used Jack rather than Clive when addressing him.
This creates a distinction between three names associated with the author: Clive Staples Lewis is his birth name, C.S. Lewis is his professional byline, and Jack is the familiar personal name. Clive Hamilton, meanwhile, was a separate pseudonym used for early poetry.