is a compound term of muga and mushin. Muga literally means no-self (derived from the Sanskrit anātman) and Mushin no-mind (also from the Sanskrit a-citta). What is negated is the empirical body-mind as an ontological independent state of existence. Muga and mushin point to the same thing - the state of egolessness - but from different perspectives. Muga refers to the negation of the physical state, mushin to the mental state of empirical existence.
To understand better mushin one needs to understand acitta- or simply its Sanskrit-root citta. Citta is not easily rendered into English. As is the case with so many other Sanskrit terms, there does not seem to be a precise equivalent for it in English. Previous translations have proposed a variety of renderings, such as 'mind-stuff', 'thinking-principle', and similar compound words. In many instances, citta seems to convey consciousness, mind, intellect or psychic mass that orders and illuminates sensations coming from without - can serve as a mirror for objects, without the senses interposing between it and its object. Thus the non-initiate is incapable of gaining freedom, because his mind, instead of being stable (still, non-fluctuating) is constantly violated by the activity of the senses, by the subconscious, and by the 'thrust for life'.
The concept of Mushin/Acitta has been traveling throughout the ages undergoing series of contextual simplifications by different philosophical and empirical systems evolving from one to another. It has arrived to the present days still embellished in the colors of mystery. Disregarding the cascade of reductionistic efforts that were meant to make the Mushin/Acitta concept more accessible to the non-initiate, the attainment of Mushin/Acitta inescapably goes through the harsh path of self-perpetuated long practice and interiorization.