C.S. Lewis’s first given name was Clive. He was born Clive Staples Lewis in Belfast, Ireland, on 29 November 1898.
Lewis disliked the name Clive during childhood and preferred the nickname “Jack,” which became his familiar name among friends and colleagues. The nickname reportedly arose after the death of his childhood dog, Jacksie, when Lewis began referring to himself by that name.
The initials “C.S.” became the form most readers encountered on his books, including The Chronicles of Narnia and Mere Christianity. Because the initials are so prominent, readers sometimes assume they conceal a different first name. They do not: C.S. stands for Clive Staples. His middle name came from his mother’s family, while “Lewis” was his family surname. “Jack” remained his personal name, but it was not his legal first name.