No elaboration other than I don’t recall seeing it before joining this board so I’m thinking it’s something local.
TMI = “Too Much Information”. It serves as a warning in a thread title that the subject matter might involve icky aliments, or, in general, what you might not want to read while eating - OR working, perhaps!
That’s…more than you really want to know. 
Haha - I meant to type “icky AILMENTS”, but, since we have had some threads abut eating/drinking unusual things - perhaps “aliments” also works.
Yeah, it stands for “Too much information”.
The one I never sussed out for myself was “WAG”
Wild Ass Guess.
Not much to add other than the fact that TMI is not a “local” SDMB thing… I am not exactly sure what the etymology of it is, but I’ve definitely been seeing it/hearing it on TV and in general for several years now.