The S. in C.S. Lewis stands for Staples.
Lewis was born Clive Staples Lewis on 29 November 1898 in Belfast. “Clive” was his given name, and “Staples” was a family name inherited from his mother's side. His full name is often shortened to C.S. Lewis in books, scholarship, and publishing.
As a child, Lewis adopted the nickname “Jack,” and people close to him generally used it for the rest of his life. The nickname came from an early family incident involving a dog named Jacksie, although the details are usually summarized rather than explored in his published works.
The combination of his formal name and familiar nickname can make biographical references seem inconsistent. “Clive Staples Lewis,” “C.S. Lewis,” and “Jack Lewis” all refer to the same writer. Staples is the specific middle name represented by the initial in his author name.