A bacronym is a word which is not an acronym but is taken to be one and for which a full form is invented by back-formation.
The formation of words using the first letters of a name or phrase is an "acronym". The word "backronym" (bacronym) is technically a created phrase that was not the actual origin of the word, whether it is an acronym or not.Example : the non-acronyms Ford and Delta form the backronyms "Fix Or Repair Daily" and "Doesn't Ever Leave The Airport.*This is similar to the use of acrostics in verse or other written forms, where the first letters of consecutive lines spell out a word or phrase.
There is NO "full form". The word diary comes from the Latin diarium ("daily allowance," from dies "day") "DIARY" is NOT an acronym or abbreviation.The phrase "DARLING I ALWAYS REMEMBER YOU" has been suggested as a bacronym, but like all bacronyms, it significantly post-dates the word and does not qualify as a "full form".