When something is slanderous, it means it is very mean and not true. It is something bad said about someone to hurt them.
Slanderous things are very bad and can cause a lot of harm to someone's reputation.
Examples:
She said some slanderous things about him.
The article was full of slanderous lies.
He was accused of making slanderous remarks about his coworker.
The gossip was so slanderous that no one believed it.
The slanderous rumors spread quickly around the town.
The slanderous words left a deep mark on her reputation.