The Jargon File (version 4.4.7, 29 Dec 2003):
[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 (18 March 2015):
(Backward acronym) A word which has been turned into
an acronym by inventing an expansion, rather than the other
way around. E.g. "ping".