C.S. Lewis used and asked friends to call him Jacksie after his childhood dog died.
When Lewis was four years old, his dog Jacksie was killed after being struck by a motor car. Lewis reportedly declared that he would no longer answer to the name Clive and would instead be called Jacksie. The name was later shortened to Jack, which became the form used by his family, friends, and colleagues.
This childhood event explains why the adult Lewis was widely known as Jack despite publishing under the initials C. S. Lewis. The nickname is especially prominent in biographies and in accounts of his friendships with J. R. R. Tolkien and the other Inklings.
Jacksie is sometimes written as Jacksie or Jacksy in secondary accounts, but Jacksie is the spelling commonly associated with Lewis’s own autobiographical account. The story comes from Surprised by Joy, his autobiography published in 1955.