A title given to a person who makes a previously cool thing uncool, including things like music, websites and blogs. People could include parents, stupid famous people, and advertisers.
"Why haven't you been on Twitter lately?"
"You didn't see? Barbra Walters made an account and talked all about how cool it was on the news."
"Wow. What a cool killer."
"I went to a party, and some cool killer just starting giving this gay speech about global warming. We all just got up and left."
"Haha, what a fag."