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 {BASIC},
   {recursive acronym} (Cygnus), {Acme}, and {mung}. Discovering backronyms
   is a common form of wordplay among hackers. Compare {retcon}.


Source: The Jargon File