A term that originated on Usenet, Godwin's Law states that as an online argument grows longer and more heated, it becomes increasingly likely that somebody will bring up Adolf Hitler or the Nazis. When such an event occurs, the person guilty of invoking Godwin's Law has effectively forfieted the argument.
"Dude, shut up. Nobody cares what you think."
"Oh, so now you're trying to censor me? Go to hell, you damn Nazi!"