It is emulating emote commands which you sometimes get in MMO computer games; for example, you might type /applaud in an MMO's chat window to make your MMO character in a game start clapping, or /sit to make your character sit down. Thus when someone types /thread, they typically mean: 'I've just proved my point so conclusively, that there is no need for this thread to continue'
In any medium where typing and reading are the primary means of communication, anyone who types TLR (too long, didn't read) might as well have typed 'I'm a thick fecker who can't read very well' lol
Also you have clearly missed the point of the subject matter. Typing "/thread" is a shorthand response, explaining it with an essay seems rather redundant, hence the use of "TLR" as comedy.
I've not missed any point, I merely answered someone's question. Typing TLR when I merely attempted to be helpful, is not only rude and uncalled for, but since the reply - long or otherwise - was not directed at you, is also an unnecessary disphemism.