tl;dr

Stands for "too long, didn't read". It can be interpreted in different ways depending on the context. 1. When responding as a first post, and only as a first post, on a thread, it means the OP's post was so long and unimportant or uninteresting that the user deemed it unworthy of his/her time. If the topic really is unimportant and uninteresting, and long enough to need more than a minute to be read and understood, the user ends up looking clever, though on some communities the user may end up looking like a troll or a lazy bum. 2. When responding during an already ongoing discussion where others haven't used the term, it means the user is trying to diminish the topic's importance or entertainment value, but considering that such a strategy won't work in a discussion where people are actually participating, the user ends up looking like a dipshit. 3. When responding with it during an argument, it means that the user in question wants to win the argument by stating that what the other person is saying is so worthless it's not worth reading. But by evading the argument instead of confronting it directly, it becomes obvious that the user got owned.