backronym

As nouns the difference between retronyms and backronym

is that retronyms is while backronym is 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.

As a noun backronym

is 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.

As a phrase fiat is

(derogatory|slang|automobiles) fix it again tony, a derogatory slang phrase for fiat, a backronym.