C.S. Lewis used the initials C.S. as his author name.
His full name was Clive Staples Lewis. The initials therefore came from his first and middle names, although he was known among friends and colleagues as Jack rather than Clive. He adopted the name C.S. Lewis for his published work, and it became one of the most recognizable author identities of the twentieth century.
The initials are sometimes expanded incorrectly because Lewis's books circulated widely in different countries and genres. They stand for Clive Staples, not for a pen name with unrelated words. Lewis used the same author identity for fantasy, literary criticism, apologetics, fiction, and devotional writing. His brother Warren Lewis generally appeared in print as W.H. Lewis, creating another initials-based name associated with the family.