it stands for "i don't spend enough time on LSD."
(hope that helps, by the way)
But I feel like with my 212 posts, I spend far too much time on this site... my girlfriend thinks I like this site more than her. Anyways, it seems people were born knowing what HTH means, but I guess god forgot to give me that gene. Since no one has ever said what it means, and always assumed that it's universal, even if I spent more hours on this site, I still wouldn'd know what it is.