C.S. Lewis’s childhood doglike companion in his private mythology was called Jacksie. Lewis gave this name to a dog that was killed by a car when he was four years old, and he subsequently used the name for himself.
After the dog’s death, Lewis reportedly rejected his given names and insisted on being called Jacksie. The name was later shortened or altered in everyday use, eventually becoming Jack, the familiar name by which he was known to friends and family for most of his life.
This episode is distinct from Lewis’s literary characters. Puddleglum belongs to The Silver Chair, Ransom is the protagonist of the Space Trilogy, and Digory Kirke appears in the Chronicles of Narnia. Jacksie is instead a biographical detail that helps explain the origin of Lewis’s lifelong nickname.