C.S. Lewis used the name Jacksie for the dog that inspired his childhood nickname. Lewis was known as Clive Staples Lewis at birth, but after the family dog died he reportedly insisted on being called Jacksie, after the animal.
The name was later shortened to Jack, and “Jack” became the personal name used by friends, family, and colleagues throughout his adult life. The familiar nickname explains why many accounts refer to him as Jack Lewis rather than Clive Lewis.
Details of the childhood story come from biographical accounts of Lewis’s early years, so the spelling is important. Jacksie is not simply a modern affectionate form of Jack in this context; it is the name associated with the dog and with the origin of Lewis’s preferred nickname. His public initials remained C.S. Lewis.