C.S. Lewis took the nickname “Jack” from the name Jacksie, which belonged to a childhood dog.
When Lewis was a child, a dog named Jacksie was killed in a road accident. Lewis reportedly decided that he would henceforth be called Jacksie, later shortened to Jack. The nickname became the form used by family members and close friends throughout much of his adult life.
His formal name was Clive Staples Lewis, but “Jack” became so established that many biographies and recollections refer to him by it. Warren Lewis, his brother, also used Jack in personal writing. The name should not be confused with a literary character or a title adopted for publication.
Lewis’s other familiar label, “C.S. Lewis,” came from his initials and was the name under which he published. The contrast between the formal initials and the intimate nickname reflects the separate public and private sides of his life as an Oxford scholar, novelist, essayist, and Christian writer.