Denpa song (電波ソング) is a Japanese slang term to describe a kind of music which often features intentionally off-key vocals, weird lyrics, and an extreme ability to get stuck in your head. It's usually extremely up-beat, happy, cute and occasionally fast. Literally, the term means "radio waves", which can describe how the reaction to the singer, actor, etc. sounds as if the person is "getting something not from this world", similar to the idea to a person wearing a tinfoil hat in Western culture.
The term Denpa was born in the 90’s to describe people who are usually day-dreaming and living in their own fantasies, and it has grown into a subculture itself, spawning many doujin units and music artists dedicated to it.