A villain's worst nightmare is the superhero who always seems to thwart his efforts, preventing him from carrying out his plans to take over the world.
Thwart is a word you'll hear in a lot of action movies, and usually it's the hero who is trying to thwart the evil plan of some super-villain. Yet even mere mortals can be thwarted in their efforts; the word simply means to prevent someone from carrying out his or her plans. An aggressive driver can thwart your attempt to snag a parking space at a crowded mall by pulling into the space before you. An aggressive shopper at that same mall can thwart your efforts to buy the last Dancing Snoopy doll by grabbing it off the shelf first.