C.S. Lewis’s familiar nickname was Jack.
Lewis’s birth name was Clive Staples Lewis, but he adopted “Jack” after a childhood event involving his dog, Jacksie. When the dog was killed, the young Lewis reportedly said that he would answer only to the name Jacksie, which was later shortened to Jack. The nickname remained with him for the rest of his life.
This is why biographies and recollections by his brother Warren often call him Jack rather than Clive. “C.S. Lewis” became the public name attached to his academic writing, literary criticism, fiction, and Christian apologetics. The initial C stood for Clive and the S for Staples, while Jack belonged primarily to his personal and social life.