C.S. Lewis's first name was Clive. He was born Clive Staples Lewis in Belfast, Ireland, on 29 November 1898.
Lewis generally disliked the name Clive and was known to family and friends as “Jack.” According to biographical accounts, he adopted that nickname during childhood after the death of a childhood dog named Jacksie. The name gradually became Jack, and it remained his familiar name throughout adult life.
The initials “C.S.” came from his legal name, Clive Staples Lewis, rather than from his better-known nickname. “Staples” was his mother’s maiden name. Lewis later became famous as a literary scholar, Christian apologist, novelist, and author of The Chronicles of Narnia. His initials can therefore obscure the fact that his first given name was Clive, while “Jack” was the name most people around him actually used.