C.S. Lewis preferred the nickname Jack among friends and colleagues.
The name originated in childhood after Lewis’s dog, Jacksie, was killed by a car. Lewis reportedly began using “Jacksie” for himself and later shortened it to “Jack.” The nickname stayed with him throughout his life.
His formal name was Clive Staples Lewis, and his books were published under C.S. Lewis. “Jack,” however, was the name used in personal correspondence and by members of his close circle, including fellow writers at Oxford.
This nickname is sometimes confused with his first name. Lewis was not legally named Jack, and “Jack” was not a pen name used for his published fiction. It was an informal personal name that became strongly associated with him.