A person who is highly stupid, perverted, sexy, ugly, smart, annoying, smelly and really anything you can think of.
To describe the reason for calling someone "lampshade" is almost impossible since it covers all aspects of what a person might be, except if the person is depressed, then he is a lamp from IKEA
Jill: What would you like to do after dinner?
Alex: Figured we could take a long walk on the beach, while watching the sunset go down. Then afterward we could lampshade.
Jill: You're such a dork, can we just skip to the lampshading?