C.S. Lewis’s middle name was Staples.
He was born Clive Staples Lewis, and the initials C.S. became the name under which he published his fiction, literary criticism, and Christian apologetics. The middle name came from his mother’s family: Florence Augusta Hamilton Lewis’s father was named Thomas Hamilton, while Staples was a family name associated with Lewis’s ancestry.
As a child, Lewis adopted the nickname Jack after the death of a childhood dog named Jacksie. Friends and family generally used Jack rather than Clive, but his books appeared under the initials C.S. Lewis.
The middle initial is occasionally expanded incorrectly because several common given names begin with S. The documented form is Staples. His full name should therefore be written Clive Staples Lewis, not Clive Samuel Lewis or Clive Stephen Lewis.