1. A type of herb that has, or is believed to have, poisonous properties or will keep something away. Often used in combination with the name of what it is said to repel such as henbane or wolfsbane. In some instances, the herb is not poisonous in the sense of it killing or sickening a person, but rather, it may induce visions.
2. In general, something that is considered bad, evil, or destructive.