A seriously lame term some chick who was trying to be cute and sell t-shirts.
To be a wonder killer means you're someone who is anal and annoying, and you can't let a question go unanswered, even if it's rhetorical. Signs that you might be a wonder killer include lack of tact and irritating know-it-all behavior.
My husband asked me "Do you think anyone still uses payphones?" and OF COURSE I had to give him a long lecture that yes, people still used payphones and it was elitist to think that everyone used cellphones! What can I say, I'm a wonder killer. Hehe!