It stands for Joanne Kathleen. She actually has no middle name, but she was requested by the publisher to decide upon two initials to go by so that boys wouldn't reject the book because it's written...
Sybill is the first name of the Divination teacher at Hogwarts; the fictional school created by JK Rowling. Almost certainly, the name derives from the Sybil of Cuma, a Greek prophetess who...
Her real name was Joanne Rowling. When she wrote her first Harry Potter book she didn't want the boys to reject it because it was written by a girl. So she made the name J.K. which stands for Joanne...