What does "Just So" mean as in the "Just So Stories"? Sometimes "Just So" might mean "just right", but that makes no sense here.
Kipling actually explains it. These were stories he told to his young children. There were some stories he told them which could be changed on each telling, but there were some stories that his daughter demanded had to be told exactly the same way each time; that is, they had to be told "just so". These are the "just so" stories.
In the "uncollected preface" to the Just So Stories, Kipling wrote
... You could alter and change these tales as much as you pleased; but in the evening there were stories meant to put Effie to sleep, and you were not allowed to alter those by one single little word. They had to be told just so; or Effie would wake up and put back the missing sentence.
It means "just because (I said so)".
In science and philosophy, a just-so story, also called an ad hoc fallacy, is an unverifiable and unfalsifiable narrative explanation for a cultural practice, a biological trait, or behavior of humans or other animals.
This phrase was popularized by the publication in 1902 of Rudyard Kipling's Just So Stories, containing fictional and deliberately fanciful tales for children, in which the stories pretend to explain animal characteristics, such as the origin of the spots on the leopard.