Definitions for backronym
A word that is either not an acronym but is taken to be one and for which a full form is invented by back-formation, or is an acronym and for which an alternative full form is invented by back-formation.
The New Hacker's Dictionary
[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.