Slang term used in Buffy the Vampire Slayer
Originally referring primarily to Spike as a self-given title, this term has come to mean the major villain of any given season. Obviously this term simply means that they are very evil and very powerful. Probably this term is a reference to The Big Bad Wolf, though it is never mentioned in the series where it comes from, so if there is a pop culture reference I'm missing, please enlighten me.
Example of Usage:
"I hate to break it to you, O Impotent One, but you're not the Big Bad anymore. You're not even the Kind of Naughty." -Xander to Spike on his inability to bite humans anymore
Examples of big bads: The Master, Angelus, Spike, Drusilla, Mayor Anthony Wilkins III, Adam, Glory, etc.