What does the S stand for C. S. Lewis?

The initials stood for 'Clive Staples' but throughout his life Lewis was referred to by family and friends as 'Jack'. The reason for this was that when he was just four years old, the family dog (named Jacksie) was run over by a car. Young Clive decided then he would only be called Jacksie - and it was only as he grew older that his mother was able to cionvince him to at least change it to Jack.