V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

NMU
       Non-Maintainer Upload (Linux, Debian)


The Free On-line Dictionary of Computing (30 December 2018):

Non-Maintainer Upload
NMU

    (NMU) A release of a Debian package by
   someone other than its usual maintainer.  E.g. "The bug was
   fixed in a recent NMU."

   (2014-09-30)