C.S. Lewis’s longtime childhood nickname was Jack.
Lewis was christened Clive Staples Lewis, but he began using “Jack” after a childhood incident involving a dog named Jacksie. When the dog was killed, four-year-old Lewis reportedly announced that he would henceforth be called Jacksie. The name was later shortened to Jack.
The nickname became his normal form of address among family, friends, fellow soldiers, and academic colleagues. Even though “C.S. Lewis” became the name printed on his books, people who knew him personally generally called him Jack.
The story is sometimes simplified by saying that “Jack” was merely a pen name. It was not: Lewis used it as a personal name throughout his adult life. His brother Warren Lewis and close friends consistently referred to him by it.