backronym

backronym – the reverse of producing an acronym; taking a word which already exists and creating a phrase (usually humorous) using the letters of the word as initials, from backwards) + acronym; see also bacronym

Brain. Hurts.

An example of a backronym from the word acronym is as follows.
Acronyms Condense Representations Of Neologisms You Memorize
In this example, because the word acronym itself is not an acronym, the phrase above is a pure backronym, not a replacement backronym. Also, because the word acronym itself appears in its backronym, the phrase is also a recursive-backronym.