What does the J.K. stand for in J.K. Rowling's name?

Joanne.

Rowling's full name is "Joanne Rowling", not, as is often assumed, "Joanne Kathleen Rowling". Before publishing her first volume, Bloomsbury feared that the target group of young boys might be reluctant to buy books written by a female author. They requested that Rowling use two initials, rather than reveal her first name. As she had no middle name, she chose K from her grandmother's name Kathleen, as the second initial of her pseudonym. The name Kathleen has never been part of her legal name.[18] She calls herself "Jo" and claims, "No one ever called me 'Joanne' when I was young, unless they were angry."[36] Her surname is pronounced like "rolling", not a rhyme for "howling" as is often supposed — or, as stated on her website: "'Rowling' (the first syllable of which is pronounced 'row' as in boat, rather than 'row' as in argument) lent itself to woeful jokes such as 'Rowling stone'."

https://en.wikipedia.org/wiki/J._K._Rowling