This exclamation can be found in a variety of anime, but it is most prominent in the series Rurouni Kenshin, wherein the main character utters it an average of three hundred times per episode. (severe exaggeration)

As near as I can tell, it is intended as a sort of exclamation of surprise/confusion/puzzlement/pain similar to "Err?" "Nyer?" or "Arrouu?" in our own dear english. (I realize these examples may be specific to my own social group/demographic, but you get the idea.)

When asked, my Japanese prof insists that it is not actually a word, which makes me wonder if she's just trying to prevent my impressionable young mind from learning slang terms. Any ideas, anyone?