It originated as a reply. Someone would make a long post, and whatever dork would reply "too long; didn't read". It became a trend, got shortened, and then peop

It originated as a reply. Someone would make a long post, and whatever dork would reply "too long; didn't read". It became a trend, got shortened, and then people started putting the summary in the OP as pre-emptive defense (and because it's useful). I want to say it started on SomethingAwful, but who knows.