C.S. Lewis’s doglike childhood nickname was Jacksie. He adopted it after a childhood dog named Jacksie was killed in an accident, and he later insisted that people call him by the shortened form “Jack.”
Lewis was born Clive Staples Lewis, but “Jack” became the name used by friends, family, and colleagues for the rest of his life. The nickname was therefore not derived simply from his initials, even though it became his most familiar personal name.
The story is one of several early experiences that Lewis connected with his developing imagination. He and his brother Warren created imaginary lands and stories during childhood, an activity that later helped shape his interest in mythology, fairy tales, and invented worlds.
Biographical accounts vary slightly in how they describe the dog’s name and the exact transition to “Jack.” The stable point is that Jacksie was the childhood nickname’s source, while Jack became the enduring form.