C.S. Lewis’s familiar lifelong nickname was Jack. His birth name was Clive Staples Lewis, but he adopted the name Jack during childhood and used it among family and friends for the rest of his life.
The nickname reportedly came from a childhood experience involving a dog named Jacksie. After the dog was killed, Lewis disliked the name Clive and began calling himself Jacksie, which was later shortened to Jack. The story is an often-mentioned detail in biographies of Lewis.
“C.S.” refers to his initials, not to the name Jack. Lewis published under his formal initials, while people close to him generally called him Jack. His brother Warren, known as Warnie, also used that name.
Lewis’s nickname is especially useful for distinguishing personal accounts from published works. Letters, memoirs, and biographies may call him Jack even when discussing the author known internationally as C.S. Lewis.