An expression, way of speaking, or quote that has come into common usage or been popularized by the show Buffy the Vampire Slayer. Examples include wig out, sitch, and adding the suffix -age to almost any word. Asking a question with an adjective followed by much is another example, as in, "Morbid, much?"
According to online forums, many people use entire quotes from the show in the daily conversation. Popular examples include "Bored, now," (said in a slow languorous voice) "What’s your childhood trauma?" and "Your logic does not resemble our earth logic."
A buffyism can also be any quote from the program that shows characters at their most witty, usually bending the English language to comic effect.
"You're a vampire. Oh, I'm sorry. Was that an offensive term? Should I say undead American?"
"To make you a vampire, they have to suck your blood. Then you have to suck their blood. It's like a whole big sucking thing."
"I'm just gonna go home, lie down, and listen to country music. The music of pain."
"Apparently Buffy has decided the problem with the English language is all those pesky words. You. Angel. Big. Smoochies?"