AFAIK and given we should leave old board shit on the old boards, it literally stands for [spoiler] The Secret Thread, which was kissinginkansas’s 2013 albums thread (or whatever year) pinned in Music.
As well as the site software not letting you unpin it, the replies count stuck for ages so no one realised there were new replies, meaning a bunch of people posted stuff in there and whole in jokes were setup a propagated. [/spoiler]
Slight correction:[spoiler] it wasn’t the post count that remained static; it was the ‘date of last post’ that remained stuck in January 2014 or something.
And yes, I think that’s where the “excited for” “excited about” in-joke came from, amongst other jokes. Can’t remember which ones.[/spoiler]