Interjection. It was once thought to refer to either the ancient Sumerian god of unpredictability, or to an obscure phrase used in conjunction with having been asked to attend a sock hop. Nowadays the word is more commonly used to create a brief, distancing moment -- what Theatre theorists refer to as a "Brechtian Moment" -- for the listener/reader. It makes ‘em go: “huh?” Basically, the word helps shake people up a little, bringing their conscious minds into a little sharper focus, even if only for a moment or two. That, and it helps bring an element of absurdism into their everyday lives. Like a non sequitur. Banana.
Peter: "What a zarching mess!"
Paul: "Yeah, it's...wait a minute, what?"