I reckon it stands for "But Surely", an excellent label for threads in which we point out the error of our ways and our logic to each other.

The thing is thgere are several types of BS thread - and they aren't all even "non-musical".

I personally think that the type of threads where people list favourite songs, and gossip about what they mean to them personally, come valiodly under the label. That's not to disparage these threads, that's what you do with friends in other settings, and it's one of the ways you get to know them better, and to broaden your appreciation of music.

But it's different from a serious discussion about something on which people may disagree, like war or religion or politics.

And it's different again from a game in which people jointly improvise and elaborate a social setting, like the Mudcat Tavern.

And it's different from an exchange of jokes and wordplay -"foolish, insolent talk; nonsense". (And I think that's got a right to be here as well.)

Using the same label for all these things doesn't make a lot of sense, and is one of the reasons why people tend not to use the prefix BS a lot of the time. I think that a few extra labels might make things go a bit smoother. Especially maybe a way of identifying the Discussion threads and the Fun Threads. (DISC & FUN ?)

At the same time, threads are going to drift whatever happens, and most of these different types of thread can run into each other. And creative and imaginative thread drifting is great fun sometimes, and the Mudcat would be a lot poorer if we all stuck to rigidly divided tramlines. So we won't.