"tl;dr" goes at the end; that's just the way it is. The "d" is for "didn't", not, for example, "won't".
I first noticed it on reddit years ago, appended to a lengthy comment. However, it seems that it may have originated in Wikipedia article discussions (according to Wikipedia...) as a shorthand _response_ to a lengthy comment. Perhaps, there was a transition period where someone wrote a lot of text, realized that the response would be a "tl;dr", and then just added it themselves instead.
I first noticed it on reddit years ago, appended to a lengthy comment. However, it seems that it may have originated in Wikipedia article discussions (according to Wikipedia...) as a shorthand _response_ to a lengthy comment. Perhaps, there was a transition period where someone wrote a lot of text, realized that the response would be a "tl;dr", and then just added it themselves instead.