Sometimes Wikiquote will link you back to Wikipedia, as I discovered whilst perusing the This Is Spinal Tap page, searching for the correct phrasing of the "these go up to eleven" scene. And on top of that discovery, an even more astounding one from the aforementioned linked article:
"Up to eleven" is in the Shorter Oxford English Dictionary.
Now, I don't understand why the article specified the SOED, because I would think that if it's in the abbreviated version, it will be in the full version, too. But who am I to argue with open-source information?
Oncemore, the reported definition is somewhat lacking: "up to the maximum volume." Pffft. Technically true, but fails to capture all the nuances of meaning in the expression. Would you define The Great Gatsby as "a book about whiny rich people?" (Well, I would, but I didn't like The Great Gatsby.) Who better than to explain the phrase than Nigel Tufnel himself:
Well, despite the glaring imperfections, I remain pleased that one of the funniest movies ever made had found such a permanent place in our vernacular.