C.S. Lewis’s lifelong nickname was Jack.
Lewis was born Clive Staples Lewis, but he adopted the name Jack during childhood and used it for the rest of his life. The nickname reportedly developed after the death of a childhood dog named Jacksie, when the young Lewis said he would answer only to the name Jack.
Friends, family, and colleagues generally called him Jack rather than Clive. The name is therefore common in biographies, letters, and recollections, even though his books were published under the initials C.S. Lewis. Those initials came from his given names, Clive Staples, while “Jack” was the personal name used in ordinary life.
Jacksie is sometimes given as the answer because it was connected with the origin of the nickname. It was not, however, Lewis’s enduring name. Jack is the short form he chose and the form used by people who knew him.