What is "up to eleven"

adv. (context idiomatic English) Beyond the maximum possible threshold

“Up to eleven,” also phrased as “these go to eleven,” is an idiom from popular culture, coined in the movie This Is Spinal Tap, where guitarist Nigel Tufnel proudly demonstrates an amplifier whose volume knob is marked from zero to eleven, instead of the usual zero to ten. The primary implication of the reference is one in which things that are essentially the same are seen as differentiated due to the user's misunderstanding of the underlying operating principles. A secondary reference may be anything being exploited to its utmost limits, or apparently exceeding them. Similarly, the expression "turning it up to eleven" refers to the act of taking something to an extreme. In 2002 the phrase entered the Shorter Oxford English Dictionary with the definition "up to maximum volume".