cloudcuckoolander

A person with extremely eccentric/odd behavior and mannerisms that are seemingly oblivious to the world around them, to the extent that it seems like they are from another planet. They are simultaneously retards and geniuses, at once both completely insane and utterly lucid. They can be very awesome, due to their general uniqueness and different way of thinking. They can also be very annoying, due to the fact that, while they are physically on Earth, they are, mentally, paying a visit to their Martian relatives.
The word "cloudcuckoolander" was coined by TVTropes Wiki.