"a history or background," especially for a fictional character or situation, c. 1990, from backgroundstory.
Entries linking to backstory
background n.
"the ground or situation to the rear of what is in front or most engaging of the attention," 1670s, from back (adj.) + ground (n.); the original use was theatrical; the word was applied to painting ("part of a picture representing what is furthest from the spectator") by 1752. The figurative sense is attested by 1854.