From Teflpedia

A backronym, or bacronym (both /bækrənɪm/) is a constructed phrase that purports to be the source of a word that is an acronym.[1] Backronyms may be invented with either serious or humorous intent, or they may be a type of false etymology or folk etymology.

The word "backronym" itself is a portmanteau and hybrid word of back and acronym.