C.S. Lewis’s surname at birth was Lewis. His full birth name was Clive Staples Lewis, and “Lewis” was his family name rather than a later pen name.
The initials C.S. stand for Clive Staples. Although he was named Clive at birth, he adopted the nickname “Jack” during childhood and was known by that name among friends and colleagues. The story commonly given for the nickname connects it with a childhood dog named Jacksie, whose death affected him deeply; Lewis later shortened the name to Jack.
Lewis’s middle name, Staples, came from his mother’s family. His brother Warren was often called Warnie, and the brothers remained close throughout their lives. The author generally published under the name C.S. Lewis, including his fiction, literary scholarship, and Christian apologetics. His name is sometimes expanded incorrectly as “Clive Staples Lewis” without the initials being explained, but both forms refer to the same writer born in Belfast in 1898.