(one's) worst nightmare

1. A situation that fills one with anxiety, dread, or fear. Often used facetiously or hyperbolically. She has a fear of heights, so being suspended over a ravine on a rickety old bridge was her worst nightmare. A: "I was stuck on my own with my girlfriend's parents for nearly three hours." B: "Oh, God, that sounds like my worst nightmare!" That's not a pet, that's a tarantula! Spiders are my worst nightmare, so get that thing away from me!

2. A person or thing that will cause one an untold amount of difficulty or hardship. As a teacher, the overbearing parent of a kid who isn't getting good grades can end up being your worst nightmare. This scandal might prove to be the senator's worst nightmare in her bid for reelection. As your supervisor, I can be your biggest advocate or your worst nightmare. Choose wisely.