I agree that the verb "hack" is the key, but I think some of the postulations are overthinking it. The basic meaning is to chop roughly; a sub-meaning, used with "into," is to gain access by chopping what's in your way ("hack" one's way into a jungle, for example). It seems reasonable that the idea of gaining access into a computer by some means other than the "front door" may have been referred to as "hacking" in - by a means requiring one to remove obstacles. Thus, the back-formation "hack," thus "hacker."

I disagree with John (from 8 years ago!). I don't know his authority on the original coinage of TLA, but it stands to reason that the A was not intended to stand for "abbreviation," as an abbreviation is primarily a shortened form of a single word (or phrase), and not what would be considered a "TLA" by most users of the term.

[Just as all acronyms are initialisms, but not all initialisms are acronyms, all acronyms and initialisms are abbreviations, but not all abbreviations are either acronyms or initialisms]

There would have been no reason to coin the term TLA for "Ala." or "Inc.", for example, as these are not fundamentally different from 2-letter or 4-letter abbreviations (e.g., Mass. and Co.). I think the coinage of "TLA" was a response to a preponderance of 3-word expressions (hence, 3-letter initialisms) in the decades of the 70's and 80's, perhaps particularly in the computer realm. "CPU," "GUI," "DOS," "RAM," for example. My guess is that the use of "A" for "acronym" was just a more recognizable word to most people than "I" for "initialism" would have been (my spell check doesn't recognize it, for example). And most people appreciate the humorous irony of "TLA" being a TLA, even if it is not strictly accurate, in either sense.

Although (being not from any part of England) I have never heard the expression, it seems fairly straightforward and apt. Incidentally, I believe that the expression would be called a "simile," rather than an "idiom." The latter being an expression that seems odd if interpreted literally, unless you are already familiar with its meaning, such as "touching base" or "bending over backwards."

For all the high brow "academics" out there - "Curriculum Vitae" is also what Playboy calls the "résumé" of the Playmate of the Month!