C.S. Lewis used the nickname Jack for most of his life.
Lewis was born Clive Staples Lewis, but he began calling himself “Jacksie” as a child after the death of a childhood dog named Jacksie. The nickname was later shortened to Jack, and friends and family generally used it instead of Clive.
The name C.S. Lewis therefore does not reflect the name by which he was usually addressed in private life. His initials came from his given names, Clive Staples, while “Lewis” was his family surname. Biographies, letters, and recollections commonly refer to him as Jack.
The nickname is also associated with the group known as the Inklings, the informal Oxford literary circle that included Lewis, J.R.R. Tolkien, Charles Williams, and Owen Barfield. “Jack” appears throughout accounts of Lewis’s friendships, teaching, correspondence, and domestic life.