The first name of C.S. Lewis was Clive. His full name was Clive Staples Lewis, although he became widely known by his initials, C.S. Lewis.
Lewis was born in Belfast in 1898 and used several names during his life. As a child, he adopted the nickname “Jacks,” which remained familiar to close friends and colleagues. The initials “C.S.” came from his given names and became the name associated with his books, lectures, and broadcasts.
Readers sometimes assume that the “S” stood for a surname or that “C.S.” was a pen name. It was not: Staples was Lewis’s middle name. His brother, Warren Hamilton Lewis, was usually called Warnie, and the brothers remained close throughout their lives.