backronym

from Jargon File (4.4.4, 14 Aug 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}.

from The Free On-line Dictionary of Computing (8 July 2008)

backronym

   <jargon> (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)