For better or worse, this term is getting more stronger then ever, even not only in anime industry, but the real world. So 電波[dempa] means an electric wave. However in the last few years, this term got another meaning though it’s not admissible in court. To get to the point, 電波 means confidential information or something should NOT come to light in public. 系[kei] means a type of person. So 電波系 means a person who tells something confidential in a public place. Nobody knows he/she does it on purpose or not because such a 電波系 does not mostly seem to read the atmosphere. So usually, 電波系 is not liked by everybody else, especially in the real world. Yes, such a 電波系 seems to be isolated or have few friends. Poor thing is the person never knows he/she is actually a 電波系 because no one tells him/her the truth…I have seen this kind of person through some anime, and every one of them is either a bookworm or such an eccentric person. Okay, I can think of Yuki Nagato(The Melancholy of Suzumiya Haruhi), Tabitha(Zero’s Familiar), or Rei Ayanami(Evangelion) now…anyone else?
One more thing, this term is newer than 電波系; it’s 地雷[jirai]. 地雷 means a (land) mine, doesn’t it? Mines are buried under the ground, so we can’t see where they are. This means a thing which should NOT be mentioned in public. When you and your colleagues and boss are having a chat in a formal place, you accidentally say something shouldn’t be mentioned in front of them(for example, about someone’s upcoming resignation or something like that). We Japanese say this, 地雷を踏む[jirai wo fumu“step on a mine“].
So, be aware of 地雷. We never know where they are.