I left it blank without any additional comment in my original round of answers. If that's not quick, off the cuff and unobtrusive, I don't know what is.
It was only after he demanded complete answers that I provided the reason for leaving it blank. I left it blank, because the nature of the question prohibits any answer at all coming to my head. Due to the definitional issues, it has as much relevance to me as being asked "what is your favorite three-sided square?" I don't have one, because I don't really know what it means.