What was C.S. Lewis's first name at birth?

C.S. Lewis's first name at birth was Clive.

He was born Clive Staples Lewis in Belfast in 1898. The initials “C.S.” came from his first and middle names, while Lewis was his family name. Despite his formal first name, he was known to family and friends as “Jack” for most of his adult life.

The nickname originated after the death of his childhood dog, Jacksie. Lewis reportedly insisted on being called Jacksie, which eventually became Jack. The name remained his preferred form of address even though it does not appear in his author name.

The middle name Staples came from his mother's family. This explains why “C.S. Lewis” does not stand for Charles Staples Lewis, a mistaken expansion sometimes repeated in informal references. His published works consistently appeared under the initials C.S. Lewis.