The Jargon File (version 4.4.7, 29 Dec 2003):

backronym
 n.

    [portmanteau of back + acronym] A word interpreted as an acronym that was
    not originally so intended. This is a special case of what linguists call
    back formation. Examples are given under recursive acronym (Cygnus), 
    Acme, and mung. Discovering backronyms is a common form of wordplay
    among hackers. Compare retcon.


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

backronym

    (Backward acronym) A word which has been turned into
   an acronym by inventing an expansion, rather than the other
   way around.  E.g. "ping".

   (2005-06-22)