I'm usually well educated with regard to these short codes, but this one had me stumped.
I did a little search on google and well, here's what it says:
LSHTTARDML = Laughing So Hard The Tears Are Running Down My Leg
I actually visualized that.. And strangely enough, it makes for an impressive pictorial of the kind of laughter in question. :)
Some people equate it to ROTFLMAO or even LOL in some cases, but they're both a far cry from the impact and imagery of LSHTTARDML. :o)
I shall use it in future, when the situation warrants it. But for now, I pay reverence to whoever came up with this chat-acronym.