What did the middle initial in C.S. Lewis’s name stand for?

The middle initial in C.S. Lewis’s name stood for Staples.

His full name was Clive Staples Lewis. “Staples” was a family name connected with his mother’s side, and Lewis used the initials “C.S.” as the public form of his name throughout his literary career.

Lewis was commonly called “Jack” by friends and family rather than Clive. He adopted that nickname during childhood, reportedly after a childhood dog named Jacksie died; the name eventually became shortened to Jack. This can make biographical references seem inconsistent, even though they refer to the same person.

The initials were especially prominent on the covers of his books, including The Chronicles of Narnia, Mere Christianity, and The Screwtape Letters. “C.S.” is therefore not a pen name: it is an abbreviation of his given and middle names.