From LQWiki

A backronym (from a portmanteau of back + acronym) is 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), and Acme. Discovering backronyms is a common form of wordplay among hackers. Compare retcon.

This article is based, in whole or in part, on entry or entries in the Jargon File.