TAGQ.

Uh, that's a good question.

Actually, my default place to look up the definition for acronyms is Google, believe it or not. The key is to know that one of the secret attributes you can use on a search is define:, so you can get the definition of any word or, yes, acronym.

For example, I recently saw a tweet that ended with "NFTU". What the heck?

A quick Google search for define:nftu and I found out that it means

"Notes from the Underbelly is an American sitcom that debuted on ABC as a midseason replacement. The series is based upon the novel of the same ...

Curious about possible meanings for ROFL? It's pretty interesting:

Notice in particular that this word / phrase / acronym matches possible definitions in other languages too, so if you're curious about what "rofl" means in Italian, say, just click on "Italian" and you'll learn that one meaning is rotolare per terra dal ridere, abbreviazione usata nei messaggi di posta elettronica e nelle chat per indicare una forte risata. .... Whatever that means.

There are plenty of other places you can look for acronyms, of course, but my first place to turn is always Google, as it is with so many other things. :-)