It stands for “too long; didn’t read”. This abbreviation’s only redeeming feature is that it contains that murmuring under-butler of punctuation marks, the semicolon. On the other hand, it announces that the user is taking time out of his or her life to tell the world not that he disagrees with something, but that he’s ignorant of it. In your face, people who know stuff! In an ideal world there would be a one-character riposte that would convey that you’d stopped reading halfway through your interlocutor’s tedious five-character put-down.
(and by the way loved “that murmuring under-butler of punctuation marks, the semicolon”)