C.S. Lewis’s first name was Clive.
He was born Clive Staples Lewis in Belfast on 29 November 1898. The initials “C.S.” came from his first and middle names, rather than representing a pen name. Friends and family often used the nickname “Jack,” which he adopted during childhood.
Lewis disliked the name Clive after a childhood incident involving his dog, which he had called Jacksie. After the dog died, Lewis said he would be called Jacksie instead. The nickname gradually became Jack, and it remained his familiar name among friends.
The name “C.S. Lewis” became the public byline attached to his theological works, literary criticism, and fiction. Readers sometimes assume the initials conceal two formal names such as Charles or Christopher, but the documented expansion is Clive Staples Lewis.