“Mu” is a traditional Zen koan which is supposed to have the power to fully awaken those who meditate on it.
“Mu” literally means “not.” When asked a question, Master Ekai would often simply exclaim “Mu!” as a way of saying that both ‘yes’ and ‘no’ were too limited to be the answer.
Mu is neither affirming something nor negating it. It is an illogical answer that points to a profound, intuitive Zen understanding, beyond the limited rational mind.
In a way, “Mu” is saying, “Unask that question!”, as in the state of full awareness, there are no questions.