Stands for "too long; didn't read".
Along with the above-mentioned use of tl;dr by lazy jerks and trolls, the use of this term is a great way to cut down long-winded, nonsensical arguments made by insane conspiracy theorists who are probably wearing tin-foil hats while typing them.
Johnny: There's no way the moon landing happened in real life! For one thing, the shadows of Neil Armstrong and Buzz Aldrin in photographs are pointing at different angles. The pictures could only have been taken in a studio! And there aren't any stars in the lunar sky in the pictures, either. How could they have ... (etc.)
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