The story
The noun was back-formed in 1998 in the episode “Bewitched, Bothered and Bewildered”, season two of Buffy the Vampire Slayer. It is a nominalization derived from the long-established adjectival pattern within Big Bad Wolf and collocative constructions inspired by it (e.g., I tried to reason with her, but to her I'm just the big bad authority figure who can't be trusted).
Etymology from Wiktionary, CC BY-SA 4.0.