The nickname of vampire William the Bloody a.k.a. “Spike” (James Marsters) on the occult adventure BUFFY THE VAMPIRE SLAVER/WB/UPN/1997-2003.
During one episode, Spike is spying on Buffy “The Slayer” Summers (Sarah Michelle Gellar) and mutters to himself, “Watch your mouth, little girl. You should know better than to tempt the fates that way. ‘Cause the big bad is back, and this time, it’s… [Suddenly he’s electrocuted by Initiative demon hunters] Urrgh! Aaaahhh!”
After the Initiative implants a violence inhibiting chip into Spikes head, he escapes. When the Scooby Gang discovers Spike’s condition Xander says “Your’e not the ‘Big Bad’ anymore. You’re not even the ‘kind of naughty’.” To which Spike says “I’m bad! It’s just I can’t bite anymore, thanks to you wankers.”
On another episode, Spike has a robotics expert create an identical, fully functioning replicant of Buffy Summers because he is smitten by her charms.
Later in the cemetery, the fake Buffy is seen sitting atop Spike writhing in sexual ecstasy and shouting “Oh Spike, you’re the Big Bad, You’re the Big Bad!”
The term “Big Bad” is a reference to the “Big Bad Wolf” in classic children’s literature.