Vicious is an adjective that means intentionally harmful or nasty. If you spread vicious rumors about a person, you're telling people things that will hurt her feelings or ruin her reputation. Cut it out!
If you're described as vicious, you're someone who does brutal things on purpose, out of ill-will towards others, like beating up defenseless puppies or poisoning the drinking water for a town. You're worse than just mean if you are vicious — in fact, you're evil! People (and puppies) will be wise to keep their distance. Other things, like gossip, animals, or cars from horror movies can be vicious, too.
