Mu is the Japanese pronunciation of the Chinese character 無 (Mandarin pronounciation wú), which means "does not have" or "without".
In English-speaking (geeky) circles, the word has become used as a reply meaning "your yes/no question has no valid answer". This usage comes from the Zen koan Joshu's dog, by way of books like Robert M. Pirsig's Zen and the Art of Motorcycle Maintenance and Douglas Hofstadter's Gödel, Escher, Bach. The koan itself is the first of 48 entries in the The Gateless Gate (無門關, Mandarin Wúménguān, Japanese Mumonkan), a collection of Zen koans published in the early 13th century. It reads, in its entirety,
A monk asked Joshu, "does a dog have the Buddha nature?". Joshu answered, "mu".
It is said that a Zen student must grapple with it for three years before arriving at understanding. Hofstadter's take on it is as follows:
Tortoise: 'MU? What is this 'MU'? What about the dog? What about Buddha-nature? What's the answer?
Achilles: Oh, but 'MU' is Joshu's answer. By saying 'MU', Joshu let the other monk know that only by not asking such questions can one know the answer to them.
Tortoise: Joshu "unasked" the question.
Achilles: Exactly!
Tortoise: 'MU' sounds like a handy thing to have around. I'd like unask a question or two, sometimes. I guess I'm beginning to get the hang of Zen.
and from there the word has been welcomed by many others who also appreciate its convenience.
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