just-so story

Short definition from Wiktionary

Noun

1. 1
   A story which supposedly explains the beginning or early development of a current state of affairs; a myth, a pourquoi story.
2. 2
   A story, especially one for children, featuring animals as characters.
3. 3
   An untestable explanation for something, such as a form of behaviour, a biological trait, or a cultural practice.