A common way for a character to announce him/herself in an intimidating manner. It doesn't matter whether he's a good guy or a bad guy, as long as the character is Badass enough to live up to this claim. That is, unless people are spoofing this line (often by the second character describing his actual worst nightmare). Compare Prepare to Die. Not to be confused with Your Worst Nightmare (which is about actual nightmares). Examples of I'm Your Worst Nightmare include: