1. acronym meaning "too long; didn't read". A response to a post that is quite lengthy.
Despite the elitist view that this is used as a way of having to get out of reading "brilliant arguments", sometimes it does make sense. Just because something is long and drawn-out does not mean it is worth reading. I've read "walls of text" that were hollow and nothing but a waste of my time. Lots of talk, little substance. In that case, tl;dr would be more than just a response to laziness.
"Chapter 1: On the usage of abbreviations in modern internet discourse. Let us begin at the beginning. It all started in 1975 with the advent of the..." *goes on for 300 pages*