C.S. Lewis’s first name was Clive.
His full birth name was Clive Staples Lewis. As a child, however, he disliked the name Clive and adopted the nickname “Jacksie,” later shortened to “Jack.” The nickname came from Jacksie, a dog killed in a road accident when Lewis was young. “Jack” became the name used by friends and colleagues for the rest of his life.
The initials C.S. Lewis therefore do not stand for Charles or Christopher. “C.S.” represents Clive Staples, while “Lewis” was his family name. His brother Warren was commonly known as Warnie, creating a household in which informal names were especially familiar.
Lewis published under “C.S. Lewis,” a form that became internationally recognized through works such as The Chronicles of Narnia, Mere Christianity, and The Screwtape Letters. His private nickname and his public author name are both central parts of his biography.