What was C.S. Lewis's full first name, including the name he usually shortened to “Jack”?

C.S. Lewis’s full name was Clive Staples Lewis. He was born Clive Staples Lewis in Belfast, Ireland, on 29 November 1898, and later became internationally known by his initials.

Lewis disliked the name Clive and adopted the nickname “Jack” during childhood. The name reportedly arose after the death of his childhood dog, Jacksie; Lewis then insisted that friends call him Jack. “Jack” remained his familiar name among friends and colleagues.

The initials C.S. therefore stand for Clive Staples, not Charles Stephen or another expanded form. Lewis used his initials professionally for works ranging from Christian apologetics to fantasy and literary criticism. He also published some books under the name N.W. Clerk, a pseudonym formed from “New Writer” and “Clerk.”