C.S. Lewis’s familiar nickname among friends and colleagues was Jack.
Lewis adopted “Jack” as a personal name when he was a child. According to accounts of his early life, a childhood dog named Jacks was killed, and Lewis began calling himself Jacks before the name gradually became Jack. His brother Warren and many later friends continued to use it.
The nickname became especially common within Lewis’s close social and literary circles. It appears frequently in biographies, letters, and recollections of his Oxford and Cambridge years. “C.S. Lewis” remained his public author name, while “Jack” signaled personal familiarity.
This distinction can cause confusion because Lewis’s full first name was Clive, not Jack. “Jack” was not a pseudonym used for his books; it was the name by which people close to him generally addressed him.