There is sometimes a confusion, or misconception, when using the prefixes of Mu and/or Nu.

Mu, as in  Mu-To  無防, Mu-Shin 無心, Mu-Dan-Sha 無段者, Mu-Nashi 無梨, refers to there being a lacking of substance, or not having the substance there in the first place (devoid). For example Mu-To means ‘No sword’, Mu-Nashi means ‘Void’ (no substance, space, etc.), Mu-Shin means ‘No thought’ (Mu; nothing. Shin; mind. No mind/clear mind), etc. Mu-Dan-Sha refers to someone who does not possess a black-belt (Mu; nothing. Dan; black-belt level. Sha; person. A person without a black-belt). 

However, the prefix ‘Nu’, as in Nu-Ki 抜気, for example, refers to the ‘release’ of energy, spirit, substance, etc. If you have Ki and then release it, or get rid of it during action of movement, this is referred to as Nu-Ki; to release the energy that you had, to let the energy go away or escape from you (maybe for reasons of recovery or movement/technique redirection, etc.). 

Further connotations: Nukeru – 抜 ‘To peel off’, ‘to come off’, ‘to be taken off’, ‘withdraw’, ‘release’, ‘leave out’, ‘come out’, ‘take away’, ‘fall out’, ‘be omitted’, etc.