When you hear someone say "turn it up to eleven," that is likely a reference to the scene in the 1984 movie This Is Spinal Tap in which guitarist Nigel Tufnel shows off his special Marshall amplifier with volume dials that goes to eleven, indicating that it's louder than anyone else's amp. The producers got Marshall to make a custom faceplate for the film.
However they came up with the idea, Marshall reportedly had no intention to mass-produce the plates and in no way anticipated the popularity of the meme, which saw them being caught out when famous and not so famous guitarists across the world began requesting amplifiers that similarly “went up to 11” just like in the film.
According to legend, whether true or not we couldn’t verify, one of the first big name artists to request such an amplifier was Eddie Van Halen who upon seeing the film, immediately called Marshall to request such an amp- a request the company apparently were happy to comply with because, well, Eddie Van Halen.
For the more every day guitarist who couldn’t get a custom-made amplifier from the factory, Marshall did begin making plates similar to the one used in the film so that amp owners themselves could make it look like their amps went to 11.
While it is the most memorable use of the joke in pop culture, a little digging reveals that Spinal Tap wasn't the first example of a dial going to eleven. And the joke itself is related to the use of 110% when bragging about something being bigger, better, or just more. That goes back at least 100 years (or maybe 110) which you can learn about at Today I Found Out.
(Image credit: Randall Munroe)