C.S. Lewis’s full first name was Clive. He was born Clive Staples Lewis in Belfast, Ireland, on 29 November 1898, and later became widely known by his initials and surname.
Lewis disliked the name Clive during childhood and chose to be called “Jack” instead. The nickname reportedly came from his childhood association with a dog named Jacksie. “Jack” remained his preferred name among friends and colleagues throughout his life.
The middle name Staples came from his mother’s family name. His initials therefore stand for Clive Staples, not Charles Staples or Christopher Staples. This is a common point of confusion because “C.S.” can sound like an abbreviated pair of conventional given names.
Lewis used several names in his writing career. Before publishing under his own name, he wrote poetry as Clive Hamilton, but his major fiction and Christian apologetic works appeared under C.S. Lewis.