denpa is crazy music with nonsensical lyrics. "denpa" literally means radio waves, stemming from the idea that radio waves can brainwash people and make them do strange things (Like a radio controlled car). The origin of denpa music (akihabara) is the electronics district in Tokyo (and is now the anime district). So it's implied that there are more denpa waves in akihabara than anywhere else. The cuter side of denpa is the influence of the anime boom from videogames in Akihabara.

Also otaku in Akihabara are said to be some of the most delusional 'denpa' otaku, so everything ties up quite nicely. I don't have anywhere to put it but I wrote an article in my spare time about denpa-kei culture and it's origins and effect on other mediums, plus its 'dark history'.