a word that was not an acronym originally but waslater amusingly or ironically changed into an imaginary one

A word re-interpreted as an acronym. In a backronym, an expansion is invented to treat an existing word as an acronym. An example is Yahoo!, now said to stand for "Yet Another Hierarchical Officious Oracle", although the founders claim they just liked the word. Often, backronyms serve a useful purpose as mnemonics. [Compound of back + acronym

A backronym or bacronym is a type of acronym that begins as an ordinary word, and is later interpreted as an acronym. The word "backronym" is a portmanteau of back (see back-formation) and acronym, and was coined in 1983. The term is also used for a new set of words put to an existing acronym (e.g. "advanced research projects agency" → ".arpa" → "address and routing parameters area").