Fruitcake

Fruitcake is internet slang used to describe someone who behaves strangely or irrationally. It is often used to refer to individuals who have unconventional beliefs or exhibit odd behavior.

Examples:

1. My neighbor is a total fruitcake, he thinks aliens are living in his basement.
2. Did you hear what Sarah said? She thinks that cats are secretly plotting to take over the world. She’s such a fruitcake!