There are multiple usages, and not one is "the correct" one. But you're right that it is very, very interesting. You are probably correct that "tl;dr" does refer to that (or as Cyn refernced below, for "too long; didn't read.") but it has, since then, mutated to the point that people actually write out "teal deer" because it references both their state of mind (being so impatient, they don't want to read other people's reactions first) as well as the physical response of slamming the key board in excitement.