Jacksie was the name of C.S. Lewis’s childhood dog, according to his autobiography.
In Surprised by Joy, Lewis describes the death of his childhood dog, Jacksie, after the animal was struck by a motorcar. The event affected him deeply and helped lead him to insist that he himself be called Jacksie rather than Clive.
The nickname later became Jack, the name by which Lewis was known to family, friends, and colleagues. “Jack” was not simply a shortened form that appeared casually in adulthood; it developed from this childhood episode and remained his preferred everyday name.
Lewis’s formal name was Clive Staples Lewis, while Jacksie belonged to the dog and then to the childhood identity Lewis adopted. These connected names are a frequent source of biographical confusion.