In conversation, a word that announces one's contempt for and hostility toward the person they're about to contradict. Most common on the Internet, for obvious reasons.
Person 1: The first movie I ever saw in the theater was "Empire Strikes Back." Man, that was intense for a 6-year-old, so much larger than life. And when Darth Vader said "Luke, I am your father," I was freaking out. I'll never forget that day.
Person 2: Um, Vader never said that line.