See The Story of Eit for the origins. It's basically an exclamation in reaction to misfortune, or an instance of misfortune itself.

Almost-antonym: "Pock", which has been translated as "the sound of the pinball machine of the universe giving you a free ball" -- basically, an unexpected small win.
As far as I can tell, it is an MIT-ism, and limited even there. I'm just doing my part to spread the meme.