An exclaimation from HomestarRunner.com. It can be found on the website's 13th menu (Atari-themed)by moving your mouse over the "email" button (Strong Bad screams it), and also by watching the first deleted scene of the Yellow Dello movie (Homestar Runner says it).
Most commonly used as a random quote, and occasionally used to tell an annoying person to get lost.
When someone doesn't know how to react in a certain given situation because of what someone has said or done. The most common signs of freaking out are when the person covers their face or hides their face.