1. A person that insists on being actively involved in a group conversation but is incapable of saying anything that adds any value.
The guy everyone wishes would just shut up and leave, because what they're talking about is actually interesting, and the constant "that's what she said"s stopped being funny about 12 minutes ago.
A social parasite that gets a little selfish boost of attention by sapping a little bit of energy out of any interesting discussion.
2. A person that does the equivalent of the above, except with the stock market.
Mike: "That's really cool, Sam, where do you work?"
Dan: "that's what she said!"
Sam: "Just two blocks away. We make the chess boards out of onyx, and marble"
Dan: "marble like my dick!"
Mike: "Neat, do you cut the stone with a water jet?"
Dan: "my dick's a water jet!"
Mike & Sam: "STFU
Dan! You are such a day trader! I hope you contract syphilis!"