Short for "Too Long; Didn't Read"
Originally intended for when someone makes a point in an arguement/discussion and the intended target of the point cannot be bothered to read the lengthy comment. However, it has been overused and brutally raped of it's meaning due to people being incapable of responding with an intellectual/witty remark. Now most commonly means "Your arguement is so sound and accurate that I can find no flaw to point out. However I do not wish to look like an uneducated fool, and so I shall instead respond as if you are the ignorant one for posting an in depth comment of such length that it would imply too much effort to read." Ironically, a large number of people would be likely to comment TL;DR to this definition if given the opportunity, thereby showing that they fall into the category of people to whom this applies
Original intended use:
An individual, most likely a "nerd", posts a comment of excessive length on a subject, unnecessarily expanding on points rather than getting to the point, thereby deserving of the response
Recent most common usage:
A point is made in which the commenter backs up their claims with facts and evidence, only to receive the response