C.S. Lewis commonly used the nickname Jack. His family and close friends generally called him Jack rather than Clive, his first given name.
The nickname grew from “Jacksie,” a name Lewis adopted after the death of his childhood dog, Jacksie. When he was young, he reportedly objected to being called by his birth name and insisted on the new name. Over time, Jacksie was shortened to Jack.
The name Jack appears throughout biographies, letters, and recollections of Lewis’s adult life. His brother Warren was known as Warnie, and the brothers remained close for most of their lives. Friends in the Oxford literary circle also used Jack.
“C.S. Lewis” was his public author name, while Jack was the personal name used in everyday relationships. The name “Jacksie” is therefore part of the nickname’s origin, not the usual answer for the name Lewis used as an adult.